I am no expert in "ELI5" so hopefully this is easy enough:
Setting up an Database connection/session is costly and "slow", There is protocol overhead, authentication, settings and so on.
If an application where to create a new Connection/Session for each Query individually this would have to happen every time, again, and again, so instead when a Session is created it is put into the Pool and reused.
I am no expert in "ELI5" so hopefully this is easy enough:
Setting up an Database connection/session is costly and "slow", There is protocol overhead, authentication, settings and so on.
If an application where to create a new Connection/Session for each Query individually this would have to happen every time, again, and again, so instead when a Session is created it is put into the Pool and reused.
also: en.wikipedia.org/wiki/Connection_pool