After I installed a single pole switch with a dimmer to an "in line" location, the light comes on when in the off position and vice versa.. I tried reversing the black supply and the black lead to the fixture, but it didn't help. What am I missing?

I installed a single pole switch with a dimmer

Does that mean just a single pole dimmer switch?

What is meant by an "in line" location.

How do you know the switch is in the off position? How many connections are there to the switch?