The call stack is a data structure used by programming languages to keep track of function calls in a program. When a function is called, a new frame is added to the top of the stack, containing information like local variables and the return address. When the function completes, its frame is removed, and control returns to the previous function. This structure helps manage the execution flow of a program, ensuring that functions execute in the correct order. If a function calls another function, the call stack allows the program to remember where to return after the called function finishes, maintaining an organized sequence of operations.