I heard it at a meal in a movie but I couldn't understand it. Can somebody tell me what it means, please?
closed as off-topic by Robusto, Em.♦, user3169, Nathan Tuggy, Cardinal Jun 7 '18 at 19:44
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The question Who's for ...? means "Who is in favor of ...?", that is, in context, "Who would like to have ...?".
To be for something in this usage means to be pro not anti.
A round is an iteration of something. In a meal context, a serving or helping.
A round can also be a slice of something, such as a slice of bread.
I'd normally expect Who wants another [round of toast, beer, etc.]? In such constructions, for approximately corresponds to is in favour of (or wants which might not always mean exactly the same thing).
There's also Who's up for another beer?, but to me that rather suggests a "straw poll" context. Perhaps the bar they're currently in is about to close, and the speaker is calling for a "collective decision" because they'd need to relocate to an all-night bar if they want to continue drinking.
Which isn't to say that OP's cited version is "incorrect", it's just less common. And whereas to be up for [something] is very much an informal usage, to me the plain form to be for [something] has significant formal connotations (in a debate, say, We are for the motion, and they are against it.)