Watch the Grace Note Slides online guitar lesson by Chris Buono from 60 Electric Guitar Techniques You MUST Know
Much like grace note hammer-ons, grace note slides have somewhat of a choked attack. It's not about the note you play as much as it's about the note you're going to. The phrasing potential is extremely valuable. Between the two techniques, you can bump your overall playing up more than a few notches once you incorporate them.
Looking for inspiration? Prasanna. He plays Carnatic phrasings on a Les Paul like no one's business. To do that, he uses copious grace note slides known as gamakas. It's otherworldly in every way. Check it out!