In addition to Bob's answer, it's a slur mark. That means the three notes are played 'in one breath' - without any more than one pluck of the right hand. That's legato playing.
As Bob says, it's usually executed by plucking the first note, hammering-on the second (one fret higher), then pulling-off with the same finger that hammered-on, back to the first note pitch.
That's the usual way to play slurs on guitar. However, another way is to slide - where the first note is plucked, then the second is slid to, then the third slid back to. That way, the transition between notes is smooth, which is, after all, what slurring is supposed to do.
So, two ways in which to play a 'slur' in guitar music. Often, in tab, it's indicated, but not always - sometimes left to the discretion of the performer.