An option sweep is basically when a big trader breaks up a large order into small individual trades. This is usually done in order to get filled fast or to conceal a large order.
The common thinking is that a sweep usually means a big trader knows something and is trying to build a position without alerting the market but that is rarely the case. More often than not, the real reason for the sweep is just to get filled quickly. Most of these large trades can turn out to be nothing more than hedges or wash trades.
Besides, there are tons of Unusual Options Activity tools that can track this activity and let you know what's going on in real time.