The concept of guitar sweep arpeggios is related to the idea of strumming a chord. When you strum a chord, you could say that you sweep your picking hand across the strings, as opposed to the idea of picking individual notes. In reality of course, even when you strum a chord quickly, you are actually picking individual strings so fast and letting them continue to ring out so that it appears as if a strum is different from individual note picking.

Instead of alternate picking the notes of an arpeggio for example where you keep changing the direction of the picking on each string and as you cross the strings, with a sweep you keep the pick going in the same direction in the same manner as how you strum a chord.

This lets you play the arpeggio very fast, and is a technique, or one way to pick an arpeggio. Some players as much as possible prefer this method of picking in general, other players prefer alternate picking in general but will still sweep pick arpeggios. In addition, how the arpeggio is fingered on the fretboard, can make it more friendly to a sweep approach, thereby being a sweep arpeggio. There is a lot more to learn about music and arpeggio playing than simply learning a key term, so it would be worth your while to get a program like Guitar Magic Evolution that gives you both the big and small view of the art of playing the guitar.