A rabbet joint is one where two parts of equal thickness are to be joined. The area of the part where the joint is to be, is cut away (rabbetted / dadoed / chiseled) such that half of its thickness is removed. The opposing face of the second part is removed in the same manner, so that when the two parts are mated, the original thickness of the parts is unchanged.
Imagine (or draw for real) on a piece of graph paper, that there are two identical "L" shapes, each 2 blocks by 3 blocks. Now put them together to create a solid block which is 2 blocks thick by 4 blocks long. That is how a rabbet joint looks and works.