Everyone has kinda hit the "Alabama = incest lol" part of the joke. What the teacher is actually going to be talking about is relative dating in an geological sense. As a very, very simple example, let's say you are trying to figure out when a volcano erupted in the past. If you look at a vertical section of earth, the oldest stuff will be on the bottom while the newest stuff will be on top. So the sediment near the top may have remains of agriculture products from humans living there, so you get an approximate date for a sample at one point. Further down, you know a bunch of fossils have been found in the area for a creature with a known extinction date. This gives you another approximate date at it's corresponding sample height.

Now you can approximate the date of any other geological events you find evidence of by comparing it to these two dates. So if there is a layer with a lot of igneous rock between the two layers mentioned, you can attempt to date the eruption relative to the other known dates. Hence Relative Dating. The same technique is also often used in archaeology, history, etc.