The in operator checks if a value exists in a list, returning True or False. Given fruits = ["apple", "banana", "cherry"], the expression "banana" in fruits returns True, while "grape" in fruits returns False.
You can negate with not in: "grape" not in fruits returns True. This is cleaner and more readable than looping to search manually.
The check is case-sensitive for strings: "Apple" in fruits returns False because the list contains "apple" with lowercase 'a'. Use membership checks for validation, conditional logic, and filtering. Any time you need to know if something exists in your collection.