Picture: On the left, you see a formula with 2 relative addresses: A2 and B2. A relative address will change when filled (copied & pasted), as shown by the changing formula on the right. In this case, we want the formula to change since we are finding different averages for each row of scores.
Picture: The cell address $B$1 is an absolute address. This cell refers to the number of eggs per carton, 12, which never changes. It is used for every formula. By making it absolute, we prevent it from changing as the formula is filled downward. The cell B4 is left relative since we do wish it to change as the formula is pasted from row to row.