a picard group is the set of isomorphism classes of invertible R-modules. I just read that phrase in the CRing project notes without further explanations:
Here are my questions:
1-under which law (I am guessing it's a restriction of a tensor product) how could we restrict it to classes? 3-why an abelian group?
I am not familiar with any algebraic geometry. Thus shall be grateful to you for using a basic vocabulary :)