Tablature is a system of music 'notation' that uses numbers instead of standard notes. It essentially comprises a diagram of the mandolin strings, with numbers relating to the frets on the neck so you know where to place your fingers.
If you imagine the mandolin neck this way up, then we can see how to correctly read the notes:
Tablature often appears alongside notated sheet music, which is a useful way to begin learning how to read notation. For more detail on this head over to the lesson on right hand picking technique.
Example tablature with notation: