(someone) got a flat

When you're talking about cars and driving, a "flat" means a flat tire: a tire that the air has come out of.

You can either say that someone "got a flat tire":

I was on time, but then I had to stop because I got a flat tire.

...or you can just say that they "got a flat":

Would you know how to change the tire if you got a flat?

change a tire

"Tires" are the rubber parts of the wheels on a car, truck, bike, or other vehicle. They're filled with air.

If one of the tires on your vehicle loses air or bursts, you have to "change" it:

I don't know how to change a tire, so I had to call someone to come do it for me.

Would (something happen) if (something happened)?

This is a way to ask a question about a situation that hasn't happened yet. It's a situation that you're just imagining. For example, imagine that you live in a place that has electricity and running water that work well. You're not sure how you would live without them. You ask a friend this question:

Would we be able to survive if we didn't have running water and electricity? I wonder...