I suspect you do not understand what "running out of RAM" means. Probably an
English problem.
Every program you execute uses some RAM. If you execute many programs, or
programs using much RAM (maybe because of memory leaks), then all the RAM may
be in use. Once a program tries to store something more, it cannot: there is
no room. That is what "running out of RAM" means.
Without a swap partition/file, the kernel kills a process as soon as a
program tries to store something but there is no more available RAM. Maybe a
program in which you have been using for hours without saving your work!
With a swap partition/file, there is still room: on the swap, i.e., on the
disk. No program is killed but the system becomes unbearably slow (because
the disk is about 100 times slower than the RAM). That lets you the time to
save your work and close the program(s) of your choice to free some RAM and
get back to a fast system.
