Pawns should be your first line of defense in the game of chess. While pawns are definitely the lowest valued pieces on the board, they can certainly cause problems when trying to keep them safe. One common and major problem that arises is the problem of having doubled pawns.
Doubled pawns can be defined as having one pawn directly in front of another one. This is the result of an earlier capture. These captures seem like a good idea at the time and may very well have been the best option at that point. The problem arises when trying to defend these doubled pawns. To further complicate matters is that these doubled pawns often become isolated pawns. Your opponent's bishops and knight very easily capture isolated pawns.
Isolated pawns are unable to be guarded or defended by another pawn. If you have two pawns in the same vertical file without an ability to guard them with each other, then you have a major weakness that will cause you problems later in the game. The best thing that you can do is to try and avoid doubling up your pawns like this. In order to avoid this mistake you must utilize careful planning and strategy from the beginning of the game. Using other primary pieces to capture attackers that your opponent sends out will help you avoid the double pawn weakness.
As mentioned earlier, pawns are considered to be the lowest value pieces on the board, but if you allow your opponent to have a piece count advantage in pawns then you are creating a weakness for yourself. If your opponent has three or four more pawns than you have, then your opponent will have an advantage.