What does the detention center mean by three walls and four nets?

Detention center with three walls and four nets is a slang term, which is usually used to describe the strict security measures of detention center. Among them, "three walls" means that there are three walls inside the detention center, and "four nets" means that there are four police nets around the detention center. This slang means that in addition to three walls, there are four nets in the detention center to isolate prisoners from the outside world. These security measures are relatively strict.