What is subjectivity?

This is a semantic question and therefore neither important nor interesting. The term 'subjectivity' can have several meanings, and you need to clarify which one you are using it with. For example, "I love so-and-so" is a subjective claim in the sense that it deals with my inner and mental world that is not exposed to other people. They cannot check it for themselves (beyond trusting my report). So you can call it a subjective claim in that sense. But if I do love so-and-so, then the claim is an objective truth because it corresponds to the state of affairs in the world that it describes. On the other hand, the claim "This picture is beautiful" is subjective because there is no possibility here of comparing it with anything in the world or in a person inside and determining that it is true (on the accepted assumption that beauty is a matter that is not rooted in the objective world). On the other hand, if I say "This picture is beautiful to me," then we are back to the claim "I love so-and-so," meaning it is an objective truth that is described by a subjective claim. "I feel that one and one make two" is exactly the same statement as "I love so-and-so." Call it subjective or objective, it's a semantic matter and not interesting.