Firstly, I apologies for the very stupid question.
I was trying to learn about differential forms on the circle, and I've come across this:
The circle is parametrized by the angle $\theta$. A differential 1-form on the circle is of the form $g d\theta$, where $g$ is smooth function on the circle.
I don't understand what is $d\theta$, it is the exterior derivative of which function ?