The way he explains it is actually pretty good.

Replacing it with actual numbers:

You live in 2020.

You go back to 2000.

In that case, events in the past (2000 to 2020) caused you to do an action (go back in time).

Once you're back there, though, time still works the same way it always did. Events in the past can cause an effect in the future, but tomorrow isn't ever going to directly change today.

So, once you're back in 2000... the events of 2020, which to you are in the past (because you remember going back in time and everything that happened before that), can change your future actions (which to you take place in the year 2000). That means that if you can go back in time to change the past, you can change the past: your actions in your immediate future can cause changes.

But, the future can't change the past. If you went back to 2000 and did something there that made you not have to do that in 2020... well, you still did do that in 2020, because how else would you have been in 2000 changing things?

The idea is that once you go back in time, any changes you make are focused on the way you experience time, not some third-party objective time.