Do SOMETHiNG other than keep asking people to answer the most basic questions for you with no hint that you are even making an attempt to find even part of the answer your self!
So here I will guide you: Google "Periodic signal"
Spend at least five minutes looking through some of the matieral that comes up in the first five results. Then come back and ask any clarifying questions you have making it clear that you have learned SOMETHING through your own efforts.