A periodic function is a function which repeats its value after a constant period ..!

Like f(x) = sin x  and g(x) = cos x  

for x =0 f(x) = 0   and g(x) = 1
for x=2pi  f(x) = 0  and g(x) = 1

So after every 2pi ...the value will get repeated...!

2pi is called as the period of f(x) and g(x) and f(x),g(x) are the periodic function ..!