Pooling is a common technique used to increase performance on games that create and destroy an object multiple times in a short period of time.
A common pooling system case is with bullets. A machine gun might shoot 30 bullets per second, which can impact performance, since Unity will have to create and destroy 30 prefabs per second.
To solve this, you can create a Pool of bullets (let's say 100) and deactivate all of them. When the machine gun starts shooting, Game Creator will pick a deactivated from the pool and use it for the shooting. Once the bullet hits the target, it's deactivated and returned to the pool to later be reused again.
Pooling in Game Creator is very easy. You just need to use the "Instantiate from Pool" Action and it will automatically generate a bunch of objects when the first instance is spawned.
By default Game Creator creates a pool of 20 instances that live up to 10 seconds. You can easily tweak these values adding a "Pool Object" component to your prefab and modifying its parameters.
If a Pool of a certain object runs out of available objects it will create new instances, increasing the size of the pool.