EQ

EQ stand for equalisation. It is the process of boosting or cutting any given frequency. Key terms associated with EQ are:

Bass

Middle

Treble

Boost 

Cut

Masking 

Instruments have the same EQ profile so are overlapping one another in your track

Muddy

There are too many lower middle frequencies in a mix of track

Frequencies you don’t want such as hiss/boom

Boom tends to be low frequencies and sometimes if the mic was too close or the sound is very bassy you might want to cut the low end

There is usually some hiss on all audio recording but it is not usually very noticeable. However, if the original recording was too quiet, so when you have to boost the volume of the track a lot in the mix. The hiss gets boosted too so then you notice it. 

Too much bass

Some instruments create a lot of bass frequencies which you might need to turn down 

Too much treble

Some instruments create a lot of high frequencies which you might need to turn down

The mic you were using

Different mics pick up different frequencies at different levels. So some adjustment after you have recorded can help shape the sound in the way you might want.