Tablature is a method of indicating the position of notes on the fretboard. There are four "tab" lines, each representing one of the four strings on the bass.
When a number is placed on one of the lines, it indicates the fret location of a note, e.g.
This indicates the 4th fret of the second string (an F# note).
This indicates the 7th fret of the 4th string (a B note).
This indicates the third string open (an A note).