A page file is a reserved space on a computer's hard drive that acts as virtual memory. When a computer runs out of physical RAM, it uses the page file to store data that is not currently being used. This helps the system manage memory more efficiently and allows it to run larger applications without crashing. The page file is essential for operating systems like Windows and Linux, as it enables multitasking by swapping data in and out of RAM. By doing so, it ensures that active processes have the memory they need while less critical data is temporarily stored on the disk.