What do you mean by page fault?

A page fault is an event that occurs when a program refers to a page that is not present in main memory. The page table entry of that page has its valid bit as 0, so the hardware raises a trap to the operating system. The operating system then loads the page from disk into a free frame (removing another page if needed), updates the page table, and restarts the instruction. A page fault costs a lot of time, because a disk access is very slow compared to a memory access.