If the present bit is set to one, it means the page is present in physical memory and everything proceeds as above; if it is set to zero, the page is not in memory but rather on disk somewhere
The act of accessing a page that is not in physical memory.

A present bit (of some kind) must be included to tell us whether the page is present in memory or not.
When not, the operating system page fault handler runs to service the page fault, and thus arranges for the transfer of the desired page from disk to memory

A Memory Page fault is an interruption that occurs when a software program attempts to access a memory block not currently stored in the system’s RAM. This exception tells the operating system to find the block in virtual memory so it can be sent from a device’s storage to RAM.
The MMU detects the page fault, but the operating system’s kernel handles the exception by making the required page accessible in the physical memory or denying an illegal memory access.