The function free() is used in the C programming language to deallocate memory that was previously allocated using functions like malloc(), calloc(), or realloc(). When memory is no longer needed, calling free() helps to prevent memory leaks, which can lead to inefficient use of system resources. When you use free(), you pass it a pointer to the memory block you want to release. After calling free(), that pointer becomes invalid, and accessing it can lead to undefined behavior. It's important to ensure that you only call free() on memory that was dynamically allocated.