Chord diagrams are quite easy to understand. Imagine you are looking at a ukulele fretboard with the headstock at the top. The strings are illustrated as vertical lines (from left to right G C E A string) and the frets are the horizontal lines with numbers to signify each fret.
All the different chord shapes are represented by dots on the strings. Each dot represents the placement of a finger onto a string. The numbers inside the dots relate to which particular finger is to be placed in that position:
(1) – Index Finger (2) – Middle Finger (3) – Ring Finger (4) – Pinky Finger
In the example on the left a G chord is shown. To play the G chord you place your 1st finger on the C string at the second fret, your 3rd finger on the E string at the third fret and your 2nd finger on the A string at the second fret.