Doubled pawns are a type of structure. The famous Andre Philidor once said pawns are the souls of chess. Doubled pawns can be great weaknesses or great advantages. Weaknesses:Can easily be blocked,are not useful in endgame,have restricted mobility. Advantages:Can create open files for rooks,control squares around them,are good shelter for kings to hide from annoying checks by queens in the endgame. Here are examples:

Those arise from the exchange variation of the Ruy Lopez. Here are when they are advantages: Hope you like it!