This Wikipedia Article defines a periodic function: Periodic function - Wikipedia, the free encyclopedia

Once you understand what a periodic function is, take a look at both of the given functions:

http://www.wolframalpha.com/input/?i=y%3Dcos^3((pi*x)/8)

http://www.wolframalpha.com/input/?i=y%3Dcos(x/2)cos((pi*x)/4)