You can still get it post jab, it’s just supposed to lessen (to a variable degree) the severity. So if you’re super high risk and catch it post-jab you might get knocked out for a week instead of being hospitalised on oxygen and maybe dying. If you’re reasonably healthy it might just be like a nasty cold and you might not even realise.
There will always be outliers where people suffer worse than they ‘should’ post vaccine, which deniers will cite as proof that it’s ineffective. You can still pass it on too, which is why social distancing is still important even after you’ve been done.
That’s how I understand it anyway.