Alternating is a fancy word for the concept of taking turns. When you have alternating serves in a ping pong match, it means you go and then your friend goes and then it's you again.
Anything that alternates can be described as alternating. An easy way to remember the meaning of alternating to think of the alter inside it: to alter means "to change," so if something is alternating, it is always changing. The seasons of the year are alternating, and so are the outfits in your closet.