Two jobs for one clause
A relative clause without commas restricts: os alunos que estudaram passaram — only those who studied. Between commas it merely explains, and applies to all of them:
| Cats, which sleep a lot, love the sun. | Os gatos, que dormem muito, adoram o sol. |
All cats sleep a lot — the clause adds a fact, it does not select. In speech the commas are little pauses.
o que resumes the whole sentence
After a comma, o que can point back at everything you just said:
| She arrived early, which surprised me. | Ela chegou cedo, o que me surpreendeu. |
The antecedent is not a noun but the whole event — the fact that she arrived early.
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