Well, say your ISP sells you 1 mb/s worth of bandwidth (don't worry about caps etc. for the purposes of this discussion). But they also sell the same to 100 other people. However, they don't actually have a 100 meg connection themselves. They've actually only got a 50 meg connection. But they know not everyone's gonna be on at the same time, and even if they are, they won't all be using their full bandwidth constantly. So they sell more than what they have. The contention ratio is how many people are "contending for", i.e. sharing the same bandwidth. In our example, it's a 2:1 contention ratio.
In the real world, you can easily get 30:1 or more. In the bad old days of SA Internet, 50:1 wasn't uncommon (cough, Mweb), but I don't think that happens anymore, although I haven't really looked into it in any depth for ages.