Sweep Picking is a guitar picking technique.
Once mastered, it allows you to play at an incredible speed. Sweeping gives a very distinctive sound. Sweep picking is usually associated with shredding.
When sweeping, you 'sweep' your pick down 2 or more strings, somewhat as if you where just strumming (so you don't pick each string individually; you sweep over them). With your other hand, you make sure all strings but 1 are muted.
If you sweep a chord, you are playing a broken chord; an arpeggio.
Also, there are so called 3 notes per string patterns. When you start on a string with an upstroke, then the third note will be an upstroke again. That way you can make a smooth transition to the next string; you just have to continue the upward motion.
One of the most skilled sweep picking guitarists is Frank Gambale. Many other guitarists who focus on speed, like Yngwie Malmsteen often use sweep picking.