Tablature, or Tab, is a system of notation which has been developed uniquely for the guitar. In short, Tab shows us which strings to play, on which frets, in which order!
In the example above, each of a the six lines represent a string: from the bottom line being the low E string, to the top line being the high E string.
So in the next example, (reading from left to right) you would play the low E string open (pick the string with your right hand, using a plectrum, and without fretting the string), followed by the G, B and e strings open, and then descend back down through them. This is an E minor arpeggio which Metal fans might recognise this as the intro to Metallica’s ‘Nothing Else Matters’.
This last example introduces some fretted notes as well as the open strings. For this example, play the low E string open, then hold down the same string behind the third fret. Repeat this on the next two strings, but fretting the strings behind the second fret rather than the third. This gives you the E minor pentatonic scale.