Compound time covers the time signatures of 6/8, 9/8 & 12/8.
(Note that forward slashes don’t form a part of a time signature on a staff but are used here for ease of reading within text)
In simple time (2/4, 3/4, 4/4) the top note of the time signature represents how many beats are in each bar & the bottom note shows what type of note will represent the beat. In simple time the beats are quarter notes or crotchets. 8th notes or quavers are notes that are shorter than the duration of a crotchet and are grouped together with a beam in clusters of 2 or 4.
In compound time, the bottom note of the time signature is represented by 8th notes or quavers. For ease of reading, the notes need to be beamed together in clusters. However unlike simple time, they are beamed in clusters of 3.
In 6/8 time, the emphasised quavers are the first and the fourth, which are the first ones in each cluster of three. This gives the impression of dotted crotchet beats. This is in contrast to simple time which has crotchet beats.