IPA: /ˈtreɪnər/
KK: /ˈtreɪnər/
A person who teaches or coaches others, especially in sports or fitness.
The trainer helped me improve my running technique.
Trainer → It is formed from "train" (from Old French *traïner*, meaning to drag or pull) and "-er" (meaning a person who). A trainer is a person who helps to develop skills or abilities, often by guiding or pulling someone along in their learning process.
Think of someone who 'pulls' or 'guides' you in your learning — that's what a trainer does.