Meaning of “main” - Learner’s Dictionary

main

adjective [ always before noun ] us uk /meɪn/
We kept to main roads all the way.Safety is our main consideration.I'll meet you at the main entrance.We usually eat our main meal in the evening.Today's main story is the hurricane in Texas.

B1 most important or largest:

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