My friend, THE POLICEMAN, just caught a thief.
Noun renamed: friend.
Step-by-step explanation:
An appositive expression is a noun or noun phrase that follows and explains another noun or pronoun.
When we have an appositive expression "not essential" we mean that said expression is not completely necessary. The sentence can be understood without it.
In such cases we use commas before and after the appositive expression.