For a variety of reasons (legal, procedural, and political) MPs can't simply resolve to reject Brexit, and didn't do so today. What they can do is reject the exit deal that May has negotiated.
Some of them have voted to do this because they want a 'softer' Brexit with a closer ongoing relationship with the EU, or no Brexit at all. Others because they think the deal is insufficiently 'hard', as it envisages some kind of ongoing relationship and they'd rather see a clean break and then deal with the consequences.
The upshot is that we leave at the end of the month with no deal unless something happens, there's currently no majority in Parliament for any of the options, and we look like international idiots.