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.
When a number is placed on one of the lines, it indicates the fret location of a note, for example:
This indicates the open 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).