Hi! It's a colloquial way of saying that tea is being made or is ready. I think you could say it for coffee or another hot drink but it would be less natural, because the word brew is referring to the tea bag brewing in the water. Hope that helps!

@Aud If "brew's on" or "brew on" means "to prepare coffee or tea", it's probably a regional dialect in British English. The most common way to say that you're preparing tea is "put the kettle on" or just "kettle on." https://www.macmillandictionary.com/dictionary/british/put-the-kettle-on

I believe it is misspelled. It should read, "Brew's on" -- i.e., "brew is on." A brew is a beverage, ususlly one that is made using heat. See also a "witches' brew" - a concoction with lots of different ingredients coming together in, usually, a negative way.