Something undeniable is clearly true: it's undeniable that water is wet and the sky is blue.
Undeniable things are obviously, clearly, totally, blatantly true, so true that it would be crazy to deny them. If you spilled soup in your lap, it's undeniable that you're wet and probably embarrassed. It's undeniable that a lot of dogs like chasing squirrels. On the other hand, it's undeniable that not everything is that simple. One person might deny what another thinks is flat-out undeniable.