A page fault is when the requested page of mem is not avail in RAM and has to be paged in (copied) from disk.
The system only keeps enough bits of various progs in RAM to keep going.
This is usually less than the sum total size of all progs (& data) needed.
Hence the rest is left on disk or stored in the special swap partition (temp extension of RAM addresses on disk).
You should google up an explanation if you really want to know about I/O, virtual mem etc.
Search for virtual memory, swap space/partitions in Linux.