The slide is a technique which involves a finger moving between two notes on the same string. The finger maintains pressure on the string, so that a continuous sound is produced until the desired note is reached. The left hand moves from one note to the next, upwards or downwards on the fretboard. The slide is indicated by a line joining two notes. Only the first note is played by the right hand, the second one is entirely produced by the left hand finger sliding up or down the fretboard. The length of the slide can be one fret or as many frets as you wish.
Practice sliding your finger up and down the fretboard. Pick only the first note. The slide is indicated on the music staff by a line leading up to the note you are sliding to. In tablature notation the slide is indicated by a line with the letter "S" above it.
This is an example of an ascending slide.
This is an example of a descending slide.