Everything has a start. In this case our start so happens to include time itself. So when we started, time started. There was nothing before that moment, like literally nothing. It’s the beginning, don’t underestimate the scope of the word nothing. There was absolutely nothing, and no time for that nothing to be. Just nothing.
You don’t have to be able to comprehend it for it to be true. There was a start, but a kind of start we cant understand, when we say “start” we actually just mean to start a specific series of events. but those events can only happen because of all events leading up to it. It matches the definition of the word start and works well in our every day speech. But when you talk about reality itself starting, its on another level, maybe you cant even use that word really.