The slide is a left hand technique which involves sliding a finger along the fretboard 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. 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, it is indicated by a line with the letter "s" above it.
A descending slide.