To test something is to verify that it works as expected. You can test just about everything and in a sense, testing is very similar to the analysis mentioned in Chapter 4, Postmortems. However, instead of coming up with a hypothesis, you are taking something that you want to exist, creating it, and then verifying that the thing you created does what you want. Be aware of the famous Dijkstra quote: "Testing shows the presence, not the absence of bugs." This is another way of saying that whatever hypothesis you test, you will not necessarily invalidate other hypotheses.
Some people ask why you should bother with testing. There is a belief that often exists, when people haven't done much programming with larger groups, that testing is pointless. This belief often comes from being told that testing is important but not seeing any immediate benefits. Testing can be frustrating and daunting, but it is not pointless because, as we have mentioned, humans are flawed. We misremember, misread...