Tablature (or 'tab' for short) is one of the most popular ways of writing music. It's like a map, showing you which strings and frets to play.
Reading tab is like looking down at the strings on your ukulele. G string is nearest to you and A furthest away.
Each note has a number that tells you which fret to play: 1, 2, 3, and so on. '0' means to play a string open, without holding any frets down. The note colors recommend which finger to use for playing the single note, e.g. yellow suggests to use the pointer finger.
In Yousician, we write chords as blocks that show you when to strum. Chord blocks have different colors to make it easier to visually separate them from each other. Chord colors do not correspond with the finger colors. The arrows in a chord block show you the strum direction: up or down. The chord diagrams at the top of the screen, show you the chord shape. The colors show the recommended finger(s) to use to form the chord and which strings to strum to play the chord.