Scansion is a way to indicate the metric pattern of poetry. It is a way to find the rhythm.
Shakespeare often wrote blank verse in iambic pentameter.
Iambic meaning it is a two syllable foot with an unstressed followed by a stressed syllable
and pentameter referring to 10 syllables making up a line.
Another way to think about this is like a heart beat: 'I am'
However, this is merely a base, a starting point. Shakespeare breaks from this pattern by adding or subtracting syllables to the line and therefore altering the scansion.
There are many reasons why Shakespeare might have broken from the scansion. It often tells the actor something about the emotional state of the character. Shakespeare gives the actors cues within the text, and we get the joy of figuring out what that means to us.
Alexandrine: 12 syllable line
Feminine ending: 11 syllable line
Empty feet: missing part of the verse
Spondee: A period in the middle of a verse line --> gives you the option to restart the scansion
Please note: This is just a brief overview of what scansion is and some of the ways it can be useful in breaking down Shakespeare text. It is not everything.