What does 90 degrees mean?

Answer 1

That one object is situated perpendicularly to another.

Telephone poles are 90 degrees to the earth if installed properly and don't lean.

When you are seated with both feet on the floor your knees are bent at approximately 90 degrees.

This answer applies to geometry rather than temperature.

Since the question is in cars and vehicles Im going to assume you are talking about torquing instructions where it says turn 90 degrees. Put your wrench on the bolt and imagine the handle is a clock arm, move it from 12 to 3 or 3 to 6. 90 degrees is 1/4 of a turn.