I saw him being rebuked in the class. In this sentence what does "being rebuked" mean grammatically?

Corrected or set straight.

It means scolded or criticized and, as a description of what occurred, is adverbial.

It is a passive voice phrase. He was being rebuked. I saw it.

It's a reduced adjective clause. "I saw him when he was being rebuked" can be reduced by taking out the subject and auxiliary "was". It's like reducing "I saw the man who was wearing a black hat" to "I saw the man wearing a black hat." In your sentence, "being rebuked" is now a participial phrase, modifying "him"

I think it's a present participle phrase. (A type of verbal phrase.) It basically acts like an adjective.

To "rebuke somebody" means to tell them that what they did was wrong (or undesirable) or what they said was wrong (or undesirable) -- in that situation. So this is (a) in the past (because the "I saw him..." says that it is past) and (b) "being rebuked" is both passive (and there it's possible to NOT include who was doing the rebuking) and continuous also (the activity was ongoing) -- as far as the verb expression goes. Did that help? Another example: "He was being rebuked by the teacher in that class". That's still both passive and continuous, but that shows (in the passive) that it was the teacher that did it. We use a "by..." phrase in the passive, but you don't always have to include that in English. Here's another: "The dog was being trained to fetch the ball." We don't know exactly who's doing that because the "by..." phrase has been omitted. Did that help?