I guess you mean the heart muscle cells, or cardiomyocytes, that were on the first Christmas Lecture.
These heart muscle cells beat because of the movement of chemical ions in and out of the cells. Individual heart muscle cells do this separately, but when there are a lot of them connected up together they will all beat in time with each other. These cells will beat together without being told to. In the body, the rate that the heart beats is controlled by electrical impulses, but in a plate, those cells beat by themselves and the rate of the beating is not controlled.
I hope this answers your question!