The word foil has several meanings:

1. A very thin, flexible sheet of metal, most commonly aluminum. Aluminum foil wraps around sandwiches to keep them fresh, covers dishes in the oven, and can even be shaped into sculptures. The foil is so thin you can easily tear it with your hands, yet it's strong enough to protect food. Before aluminum became common, people used tin foil, which is where the phrase “tin foil hat” comes from.
2. To stop someone's plan or prevent something from succeeding. If a group of students tries to sneak extra dessert from the cafeteria but the lunch monitor catches them, she foils their scheme. In adventure stories, heroes often foil the villain's evil plots at the last minute. A foiled attempt is one that failed because someone or something got in the way.
3. A character who contrasts with another character to highlight certain qualities. In Charlotte's Web, Templeton the rat serves as a foil to Charlotte: his selfishness makes her kindness and wisdom stand out more clearly. When two characters are foils to each other, putting them side by side helps readers see each one's distinctive traits more vividly.
4. A thin, light sword used in the sport of fencing. Fencers duel with foils, trying to score points by touching their opponent with the tip of the blade.