In JavaScript, a closure is a function that "closes over" the variables from its outer scope, even after that outer function has finished executing. It allows the inner function to retain access to its enclosing function's variables. Closures are crucial in front-end development because they enable data encapsulation, private variables, and the creation of modular, reusable code. For example, in event handling, closures help maintain state between events, ensuring that the correct values are used when a callback function is executed. This concept is fundamental when working with asynchronous operations, such as handling user interactions in web applications.