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 guitar. The thickest string is nearest to you, and the thinnest one is 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.
For example, in many chords, you shouldn't strum the big E-string (the thickest one). When this happens, the string is marked with an "X" on the chord diagram. You can avoid strumming that string, or use your fingers (or thumb) to mute the unwanted strings by touching them lightly.
For more information about tablature, check this video.