Thickness has to do with density.
Coarseness has to do with strand density
If you had thick hair, but fine strands, it would be like a forest packed with skinny trees.
If you had thick hair and coarse(fat) strands, it would be like a forest packed with wide/fat/thick/ trees.
Thin hair would be like a forest with few trees per square meter.
... there is also medium thickness and medium strand diameter.
To determine the thickness of your strands I heard you can compare a single strand to thread.
Coarse: strand is as thick or thicker than a piece of thread
Medium: strand is about as thick as half a piece of thread (like if you unravel the thread into two equal sections)
Fine: strand is about as thick as a quarter of a piece of thread... or thinner.
I hope that helps, I'm sure someone else will be able to add more
Lys