I was studying English, so I downloaded a comprehension reading test and I found a phrase which I can’t understand:
Seat belts save lives and that’s a fact. That’s why I don’t drive anywhere until mine is on tight.
I think that it’s an idiom because I don’t find word definitions that help me understand the phrase.
It’s not an idiom. Each word means exactly what it says.
Mine means ‘my one’, referring back to the seat belt.
On tight means exactly what it says – fitted securely. So
I will not drive anywhere until my seat belt is fitted securely.