“I just barely made it back in time to return the car.”
You rented a car. You were supposed to return the car by 6:00 pm on Sunday. You returned it at 5:50. You say this as you're talking to your friend about your trip.
I just barely made it back in time to return the car.
When you succeed in doing something but come really close to failing, you can say that you "just barely" did it. In the example:
I just barely made it back in time to return the car
...you did return the car on time but you almost did it too late. Another example:
I took French in high school, but I just barely passed it.
This means that you got a low grade which was passing, but close to failing.
To "make it back" to somewhere means to successfully return to your home or to the place that you left from:
Hasn't Doug made it back yet?
You use the phrase "___ in time to ___" when you're talking about an action that you have finish before a certain time, in order to do something else. For example:
In this example, the action you have to finish is "make it back home". You had to do this by a certain time in order to watch the last episode of the TV show Breaking Bad.
When you borrow or rent something, "returning" it means giving it back to the person who owns it.
Have you returned those DVDs to the library?