The Plate Tectonics Theory describes the fact that the earth is broken up into multiple parts called plates, which constantly change shape and size.
A picture of the plates and the names on planet earth.
Some people might say that Plate Tectonics is the same thing as Continental Drift, but they're not. Plate Tectonics describe plates in general, while Continental Drift describes the change in the plates over time, from Pangaea to the location of the continents now.
A group of pictures demonstrating and explaining Continental Drift.
Some evidence supporting the the theory is that there are mesosaurus, glossopteris, and cynognathus fossils on both south america and africa which are not close enough for the dinosaurs to travel between the two continents so at one point they must have been together.