It depends on how the tabber means it. Either a palm mute or a string that you skip. If you dont know how to palm mute: Just take your palm and rest it across the bridge and slightly on the strings so you hear the fret being muted not just a dud. The more you put your hand twards the nut, the less notes you'll hear, the closer you are to not have the strings muted, the more you'll hear. If that don't help go here. What song you learning?