“We took turns driving, so it wasn't that bad.”
You and a friend are visiting your hometown, which is far away from where you live now. You drove there, and it took 11 hours. Someone asks you how long the drive was, and you tell them. They think it seems like a really long drive, but you say this because you didn't mind it.
We took turns driving, so it wasn't that bad.
When you "take turns" doing something with someone, it means that one person does it for a while, then the other person does it, then back to the first person, and so on.
We have a weekly meeting at work, and we each take turns leading it.
Me and Erica took turns taking pictures of each other with our new camera.
When something is not bad, or not very bad, you say it's "not that bad". You can use this when something isn't as bad as you expected it to be:
I had a cup of coffee from McDonald's, and it wasn't that bad.
You also use this when someone else thinks that something is bad and you want to reassure them. Like when a child is about to get a shot at the doctor's office, you can say:
Don't worry, it's not that bad. It'll only hurt for a second.