Recursion is a technique in computer programming where a function calls itself repeatedly in order to solve a problem. The function needs to have a base case, which is a condition that stops the recursive calls and returns a result, and a recursive case, which makes the function call itself with a simplified version of the original problem. The idea is to break down a complex problem into smaller, similar sub-problems until it can be solved easily. Recursion is used in many algorithms and data structures, such as tree traversal, divide-and-conquer, and backtracking.