In British English, a crisp is a thin slice of potato, fried or baked until crunchy and eaten as a snack; it's the British term for what American English calls a potato chip (British "chip" instead refers to what Americans call a french fry).
Popular flavors vary by region: salt & vinegar and cheese & onion in the UK; barbecue and sour cream & onion in the US; seaweed and wasabi in parts of Asia. According to a popular but historically disputed legend, the potato chip was invented in 1853 in Saratoga Springs, New York, after a diner complained his fried potatoes were sliced too thick.
Crushed crisps/chips are also used as a crunchy topping for salads, soups, and casseroles, or as a coating for fried foods.