The name of the playground

That's right. But the usage is different. In addition, playground refers to children's playground. Playground English should be a field/training ground.

When we say "I'm going to the playground", let's assume that the other party already knows which playground I'm going back to.

We said "I'm going to the playground" without presupposing that the other party knew which playground I was going to. So the other party may ask "which playground (which playground? )"。