Watch the Tremolocity online guitar lesson by Jeff McErlain from 50 Blues Guitar Licks You MUST Know
Tremolo picking basically means, PICK AS FAST AS YOU CAN! This lick is borrowed from SRV. I highly suggest you watch clips of him playing. His physical approach to the instrument has a huge effect on his tone and attack. That is true for all players. A SRV lick won't sound right unless you attack the instrument the same way. Oh, by the way, you can attack the strings softly as well.