IANA cosmologist, but here's my current view on it.

We perceive time as having a particular direction. This arrow of time is not dictated by any known physical laws. That is, if we look at all events and interactions in the Universe, exactly the same rules apply whether played forwards or in reverse order.

Any explanation of the arrow of time boils down to one of entropy. As time passes, the Universe gradually goes from a state of greater order (less entropy) to one of less order (greater entropy). Think of the Universe as moving gradually and somewhat randomly from each state to the next. There are exponentially more unordered states than ordered ones, thus from any somewhat ordered state, there is a much greater chance of moving to a less ordered one than to a more ordered one.

So we can try to imagine going backwards in time, with more and more order and less and less entropy the further back we go. Eventually we will reach a point where the universe is perfectly ordered, with minimal entropy. This would be the beginning of the Big Bang and the beginning of time itself.

Asking what would be before the beginning of time is a bit like asking what is north of the North Pole. You can keep going straight towards the North Pole, but at the point where you reach the North Pole, you cannot get further north. But you can keep going in the same direction you were heading. You will find yourself going straight south, although still going away from where you initially started.

Similarly, we can imagine going backwards in time, and then keep going as we reach the Big Bang. Suddenly we would find ourselves going forwards in time, but not going towards the point we started from. We'd be in a sort of mirror universe, with the mirror being the Big Bang.