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 bass. E string is nearest to you and G 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).
In Yousician, the color of the note tells you which finger to use. Check out the little hand to see how it works.