Present/absent Bit

The present/absent bit represents a boolean value, if the bit is 1 the request virtual page is currently stored inside a physical page frame, to the page is "present". But if the bit is 0 the requested page is currently not stored in memory.

What happens if a page is currently missing in memory?

A page fault... Which corresponds to an Interrupt, which the operating system has to handle. After running this page fault interrupt routine, the required logical page should be placed inside a physical page frame and the program can continue to execute.