There is no starbucks union... yet. One store in buffalo has unionized and hopefully more follow.

loaded question

this is gonna be a long post lol, get ready

so, there isn't a "starbucks union" (yet). you've heard about the buffalo store that's unionized, i'm thinking. those partners are represented by the union workers united.
basically, this means that it's very much possible for other stores to unionize. this is a big deal because retail/food service has aways struggled to unionize, especially powerful megacorporations like starbucks.
what this does for baristas (and unions in general) is basically giving them more power. 1 barista, or a few, have no power at all compared to the absolute behemoth of a corporation like sbux. the idea of a union, however, is that all these workers are all sort of working together in order to counteract that power imbalance using numbers. 1 barista holds no power like i said, but what about 20,000 baristas working and standing together? 200,000? 2 million?
the partners at the unionized store can now negotiate ("collective bargaining" is the term you'll hear used a lot) with the company for things like wages, working conditions, how benefits work, etc., instead of partners having really no say in that normally. starbucks (and partners) are then bound by contract to honor these negotiations when they've both agreed upon them.
basically what all this entails (in theory, because yes, unfortunately corruption and just plan inefficiency does exist sometimes) is that baristas will be better protected from unfair conditions and treament, have more say in their workplace, and be able to have their voice heard as an equal.

i hope this makes sense, let me know if i missed anything! also i'm very much not an expert so don't quote me on any of this haha :)

Unions in theory are supposed to be a buffer between employers and workers. Collective bargaining is a big deal since generally it leads to higher wages and better working conditions. But they are notoriously difficult to set up, maintain, and utilize.