A slide is when you keep friction between your playing/fingering hand, and the string, as you slide up the fretboard. The notation for this technique is shown below. First, pluck the note (although this is optional, the action of sliding itself can create some vibration in the string), and then simply hold it down and slide your left hand (if you are a right handed guitarist) or right hand (if you are a left handed guitarist) around the fretboard.