delta represents the amount of time elapsed during one frame. If your velocity is in units of distance/time (such as "100 pixels/second"), then multiplying by the frame time will result in the distance to be moved during one frame.
IE, if I have velocity of
120 pixels/sec and delta is
1/60 sec, then each frame I should move 2 pixels, and after a full second, I'll have moved 120 pixels.
For a much more detailed explanation, with examples and animations, see here: