Can someone explain Tyre Management?

So this is pretty much my first ever F1 game.

I've played a couple racing games in the past but those were no where as tactical as this.

Right now I'm doing my first couple career training sessions, and I'm really struggling with the Tyre Management ones. I know nothing, and I mean absolutely nothing about cars, racing or Formula One.

I feel like they didn't really explain what I'm supposed to do. I know I'm supposed to stay in the green or purple bar, but no matter what I do it just goes straight into red.

Can someone please explain what I'm actually supposed to do here? Keep in mind I'm using an XB1 controller.

Thanks!