Stonehenge must have been an impressive meeting place for visitors from by far-flung tribes.
Even so, most academics and scientists now agree that it was also the Bronze Age equivalent of an astronomical observatory.
The Bronze Age people were trying to chart the apparently random movements of the heavenly bodies, and to predict the equinoxes and solstices that were so important in their lives, so that they could gain a greater understanding of the universe they lived in.

Just a few of the sightlines shown in the diagram above:-

• Summer solstice sunrise viewed from the centre of Stonehenge through the Heelstones.

• Summer solstice sunset viewed from Stone G to Station Stone North.

• Winter solstice sunrise viewed from Station Stone North to Stone G.

• Winter solstice sunset viewed through the largest trilothon.

• Equinox sunrise viewed from Station Stone North to Stone C.

• Most southerly moonrise viewed from Station Stone North to Station Stone East.

• Most northerly moonset viewed from Station Stone South to Station Stone West.

• Equinox moonrise viewed from Station Stone North to Stone B.

This is almost the most impressive feature of Stonehenge, since it becomes obvious that the theories and calculations for predicting lunar and solar eclipses were known to Bronze Age people before any of the stones were even brought to the site, 5000 years ago!
This knowledge was completely lost to us until the work of Copernicus and Kepler, 4600 years later!

In 1663 John Aubrey carried out the first systematic inspection of the Stonehenge site, and the Aubrey Holes are named after him.

The holes are a metre wide and deep, and were first thought to be 'post holes' for wooden posts.
After human bones were excavated it was seemed that they were intended as graves, but it is more likely that a venerated foreman or official of the site was buried there in order to honour him.

In the 1960s the American astronomer Gerald Hawkins was the first to propose that the Aubrey Holes were used in the calculation of sun and moon eclipses.

Astronomers now agree that the method later proposed by Sir Fred Hoyle works correctly and accurately:-

The SUN stone is moved (anticlockwise) by 1 Aubrey Hole every 6 1/2 days.
(6 1/2 x 56 = 364 days).  Errors can be corrected at midsummer and midwinter.

The MOON stone is moved (anticlockwise) by 1 Aubrey Hole twice per day.
(One cicuit would be completed in 28 days, a lunar month.)
Errors can be corrected by skipping holes in rings at Station Stone 92 or 94.

The NODE stones are moved (clockwise) by 1 Aubrey Hole every 4 months.
When the SUN and MOON stones are together or opposite, and the NODE stone is in a SUN or MOON hole, then there will be an eclipse.

The SUN, MOON and NODE stones could originally be synchronised at the time of an eclipse.

...a Solstice?

Solstice is a Latin (Roman) word meaning 'sun standing still'.
This term refers to the time of year when the Sun appears to stand still in its daily movement along the horizon.

There are two Solstices each year, at Mid-Summer and Mid-Winter.

Summer Solstice 20th ~ 23rd June

The Summer Solstice occurs on Mid-Summer's Day when the Sun reaches its most northerly point, marking the longest day and the shortest night in the Northern Hemisphere, before it begins its daily march back toward the East for Autumn.

Winter Solstice 20th ~ 23rd December

The Winter Solstice occurs on Mid-Winter's Day when the Sun reaches its most southerly point, marking the shortest day and the longest night in the Northern Hemisphere, before it begins its daily march back toward the East for Spring.

...an Equinox?

Equinox is the Latin (Roman) word meaning 'equal night'.

This term refers to the time of year when the length of the day is equal to the length of the night (12 hours) and is the mid-point between Mid-Summer and Mid-Winter (the Solstices).

There are two Equinoxes each year (or 'Equinoces' to use the correct Latin plural).

The Spring Equinox occurs at the mid-point between Mid-Winter and Mid-Summer, as the Sun appears to rise each day further and further North.

The Autumn Equinox occurs at the mid-point between Mid-Summer and Mid-Winter, as the Sun appears to rise each day further and further South.

...an Eclipse?

An 'Eclipse of the Sun' happens when the Moon moves between the Earth and the Sun, so that the Sun's light is obscured, and a shadow is thrown onto the Earth. 

An 'Eclipse of the Moon' happens when the Earth moves between the Sun and the Moon, so that the Earth's shadow is thrown onto the Moon.

 Either of these occurences would have been quite terrifying to primitive peoples, and so the Bronze Age people would understandably have been very keen to gain the skill of predicting eclipses, in order to feel more in control of them, or at least to gain some understanding of the process through repetition and measurement.

 ...Stage I/III?

STAGE I

Stage I in the construction of Stonehenge was the period from 3100 BC to 2910 BC.  During this time the enclosing Earth Circle was dug, the Heel Stones and the Altar Stone positioned, and the Aubrey Holes dug.

 STAGE II

Stage II lasted from 2150 BC to 2100 BC, and during this period the Blue Stones were placed and the Station Stones positioned.

 STAGE III

Stage III from 2100 BC saw the construction of the great Sarsen Stone ring and the inner Trilothons.

The Stages shown in 'How Was it Built' refer to the sightlines which were possible at the time of Stage I/II (the Aubrey Holes and Station Stones), and then a second sightline after the Sarsen Stone ring and Trilothons were erected during Stage III.