A relative clause gives more information about a noun.
Essential or extra
Defining (no commas) identifies which one: the keys that open this door are missing.
Non-defining (commas) adds a bonus: these keys, which are old, still work.
Commas for extra
My brother, who lives in Rome, is a chef. Drop the clause and the sentence still stands. Defining clauses you cannot drop without losing which one.
The keys that open this door, are missing. — extra commas on an essential clause
The keys that open this door are missing. — yes