Watch the Rake online guitar lesson by Chris Buono from 60 Electric Guitar Techniques You MUST Know
A rake is an ornamental picking technique where the two to four strings below the note you're intending to play are muted in some fashion to create a scratchy, percussive technique. The intention is to give the note a more pronounced attack and to help make it pop. This technique is not to be taken as a form of sweeping picking where you glide the pick across the strings to more economically play one-note-per-string instances. When raking, you're not looking for pitch, rather you're looking for effect. All that said, be sure to keep your raking at bay. Like any embellishment technique, less is more.
While raking is common to any jazz player's bag, you'll hear this technique employed by everyone from SRV to EVH.