What does "three R's" mean?

The fundamentals of education, as in It's a terrible school; the children are not even taught the three R's. It is widely believed that Sir William Curtis, an alderman who became Lord Mayor of London, once presented a toast to the three R's—reading, riting, and rithmetic—thereby betraying his illiteracy. In any event, the term was picked up by others and so used from the early 1800s on.