A radian is a unit used to measure an angle of a circle.
I would explain it using 2pi to equal the whole circumference of a circle.
Unit circles are labeled in radians.
The circumference of a circle is 360 degrees or 2pi
Radians relate to degrees because they are just different units of measure to measure the same thing.
I prefer degrees because they're easier to understand. Degrees are more "pure" numbers.