When writing music down on paper for other musicians to play the music is divided into what is called measures. The measures are divided into vertical lines. The lines that musical notes are placed on are called a staff.
Can you see the vertical lines in the music staff below?
Same thing here in this tablature staff:
Note: The funny looking symbol at the far left is a treble clef. This is just signifying that the music staff is for a treble clef instrument (which the guitar is), but it is of no concern to us at this point.