Website traffic is simply the flow of visitors coming to a site — how many people open it, how often, and which pages they look at. When people say a site has “a lot of traffic”, they mean many visitors are coming to it.
Think of foot traffic in a shop: the more people walk through the door, the more chances there are to make a sale — but a very busy shop also needs enough staff and space so it does not become slow and crowded. A website is the same: more visitors is good, but the site must be able to handle them without slowing down.
Handling lots of traffic well depends on solid Hosting and often a CDN, which spread the load so the site stays fast even when many people arrive at once.