The common area is property intended for the shared use of all community members. Depending on the type of association you live in, the common area may be owned by the association or the owners. Typical components of common areas include, but are not limited to, pools, tennis courts, playgrounds, parks, hiking trails, etc.
In a nutshell: Common areas are your community’s sandbox. No one can lay claim to the sandbox – it’s for everybody! Though, typically, you need to be a member of the community to use the common areas. So, I guess it’s like a country club sandbox. Do they have those? I bet they’re fancy. Anyway, as is true of universal sandbox etiquette, you, too, should be respectful of others while playing in the common areas.