Tablature is a method of indicating the position of notes on the fretboard. There are six "tab" lines each representing one of the six strings on the six strings on the guitar. When a number is placed on one of the lines, it indicates the fret location of the note, e.g.:
This indicates the seventh fret of the 5th string (an E note).
This indicates the second fret of the 3rd string (an A note).
This indicates the 3rd string open (a G note).
This indicates the first three strings are played together.