Meaning of “main” - Learner’s Dictionary

main

adjective [ always before noun ] us uk /meɪn/

B1 most important or largest:

the main problem/reason
The main airport is 15 miles from the capital.

More examples

the main thing

B2 the most important fact in a situation:

You're happy and that's the main thing.

(Definition of “main adjective” from the Cambridge Learner’s Dictionary © Cambridge University Press)