It means that I am not interested in hearing what you think. Your opinion doesn’t count. Your opinion doesn’t matter. I’m not interested in what you think.

And just to add . . . "you don't get an opinion" is even a bit more rude, and more condescending, than "You don't have a say." "You don't get an opinion" (which is also not correct English, though it is said that way) suggests that the person doesn't even have the right to think a contrary thought; "you don't have a say" means that the person's opinion, whatever it is, doesn't affect the decision.

It’s rude but you can say it. You can also say it jokingly or to a child.