Sliding, not to be confused with 'slide guitar', is a simple technique used to get from one note to another on the same string by sliding your finger along the string. You can slide up or down in pitch. The main thing to remember is that you should keep good firm contact with the fingerboard or else you'll lose the note, it will choke off. The whole thing relies on retaining the vibration of the initial pluck on the first note.
It's most commonly used when playing single notes, but you can also slide double stops (two notes played together on different strings) up or down and you can even slide whole chord shapes up or down. That means that you can only slide between two chords of the same flavor of course. The movie does a much better job of showing it all than trying explain it in words.