Realloc is a function in the C programming language used to change the size of a previously allocated memory block. It allows programmers to resize an array or memory segment dynamically, which is useful when the amount of data being stored changes during program execution. If the new size is larger, Realloc may allocate a new memory block and copy the existing data to it. When using Realloc, it is important to handle the return value properly. If the function fails to allocate the requested memory, it returns a null pointer, and the original memory block remains unchanged. This requires careful error checking to avoid memory leaks or data loss.