IBM paid a ~30% premium on the current stock price, so all shareholders (I imagine employees own a bunch of shares) will get a decent chunk of cash.

Some redundant departments (HR, finance, accounting and the like) will be downsized after the acquisition.

Engineering and product will be unaffected in the short term, but in a year or two the IBM culture will start to seep in, and that would be a good time for tenured employees to start planning their exits. That's also when lock-up agreements will expire and the existing leadership of Confluent will depart and be replaced by IBM execs.