frameCount is a system variable, indicating the number of frames that has been rendered since the sketch started running

frames.length is the number of elements in the array  called frames.

% (Modulo) "Calculates the remainder when one number is divided by another."

E.g. 10 % 5 = 0, since there is no remainder.
10 % 6 = 4, since 1 * 6 + 4 = 10. 10 can only be divided by 6 one time, but leaves 4.

The result of doing frameCount % frames.length, is that when frameCount is equal to frames.length, it will give 0, and thus starting from the beginning of the sequence.