one who trains other persons or animals
"The horse trainer adjusted the saddle before leading the animal into the ring."
simulator consisting of a machine on the ground that simulates the conditions of flying a plane
"The airline pilot practiced emergency landing procedures in the flight trainer before taking off from the actual runway."
A person who trains another; a coach.
"The tennis trainer adjusted her grip to help her improve her serve."
In plain English: A trainer is someone who teaches you how to do something better by giving advice and practice.
"The new running trainer helped me improve my speed before the race."
The word trainer comes from combining train with the agent suffix -er. It originally referred to someone who trains animals or people but has come into English directly through this straightforward formation without a shift in core meaning.