a restricted area

A "restricted area" is a place that the general public isn't allowed to enter.

Most public buildings have "restricted areas" where only employees are allowed to enter, or only cleaning staff, only administrators, only security guards, and so on. There are also "restricted areas" at outdoor events like concerts and festivals.

Do not enter

This is a warning that's written on signs. It means that you can't go into a place.

Usually, "enter" requires an object:

I saw them enter the room.

But signs are written to leave out as many words as possible. In this case, "Do not enter" means "Do not enter (this door, this area, etc.)."