If you drive during rush hour, you're likely to hit some heavy traffic, and if you get stuck in a big traffic jam, you'll definitely be late. It's not just cars that create traffic — a large number of anything trying to occupy the same space adds up to traffic.
Internet providers may see a lot of traffic when there's big news and everyone is trying to sign on to find out what happened. Pedestrians also create traffic, like those walking past your new burrito palace and pet costume shop. While your business may be perfectly respectable, you would want to watch out for people who traffic stolen goods and illegal drugs. In that sense, traffic acts as a verb indicating you're trading something, often illegally.