Instances amount on Roblox renderer stats

Im not sure on what category this belongs to so i just put platform usage,

The Instances amount right here

What does it mean?
Also whenever I add parts to the game, the instances amount goes up and when i delete that part, the instances do not go down, im just confused about this, and how it can be used to optimize, or whatever it is, anything helps!

The instance count is the total amount of instances in your game. It should be a very large number but should not exceed 20 million.

Because you kept it in memory. Calling Destroy on it might make it go down.

but the thing is i stop the game and i replay it so there are no parts