IPA: /ˈmeɪnli/
KK: /meɪnli/
In large part or mostly; primarily.
The project was mainly focused on improving community health.
Mainly → It is formed from "main" (from Old French *main*, meaning principal or chief) and "-ly" (meaning in the manner of). The word "mainly" means in the manner of being principal or chiefly.
Think of something being the 'chief' or 'principal' part — that's why 'mainly' refers to the primary aspect.