Simply put - a pump is a device that moves fluid - generally from a lower level to a higher level against gravity.
To do this - the device must already be submerged in the fluid, since it cannot pull up the fluid from a lower level to its own level and then discharge it to a higher level. This will require it then to suck air through an inlet pipe (submerged in the fluid) to create vacuum that will then pull up the fluid to it's own level.
Pumps that cannot do the air sucking - needs, what is called priming.
The inlet pipe is fitted with a one-way (up - only) valve, called the foot-valve, fitted at it's mouth which is submerged in the fluid. The pipe then is filled from outside with the same fluid till it tops up till the pump. The foot-valve prevents the fluid from draining away (up-only). This procedure is called "priming the pump".
Once primed, the pump remains submerged in the fluid all the time unless the foot-valve leaks. Starting the pump in this condition allows the fluid to be "pumped" through the discharge pipe and all is well. Priming is done only once, till the foot valve leaks.