A 'trailer' is a thing on wheels that is pulled by a car or truck (or animal).
A 'tractor trailer' is a big truck that is pulling a big trailer.
It is also called an '18-wheeler' because it has 18 wheels.
This blog is mainly to explain a different English language expression every day. Sometimes I post other stuff. I also have a YouTube channel which is mostly about computer software: https://www.youtube.com/englishspacedog
