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 of the guitar. Study the following diagram.
The left hand fingers are numbered as such:
When a number is placed on one of the lines, it indicates the fret location of a note e.g.
This indicates the open (unfretted) 3rd string (a G note).
This indicates the 3rd fret of the 5th string (a C note).
This indicates the 1st fret of the 1st string (an F note).