What does it mean for something to be socially constructed?

I thought I'd write a quick post about this, because someone's search engine query brought them this way, and I don't think that anything I've written so far really answers it.

The way that I've encountered the concept of social construction is that, basically, things like rocks and humus and earthworms are not socially constructed, while things like washing machines, gender, and sex are. What supports a social construction is a a society or culture's defined conventional rules.

I'm at a point in Foucault's History of Sexuality where he is talking about the creation of homosexuality. Essentially, he notes that before the Victorian (well, modern? I'm not sure) era, sexualities were not compartmentalized like they are now, and that until homosexuality was defined as "someone who has an (unnatural) desire for someone of the same sex" that it actually became homosexuality.

We created homosexuality, then--it's a social construction. As sexuality shifted, and we added more rules, we added more quantifiers of identity that break the rules. Ta-da! Homosexuality.

God, that doesn't really answer the question of what does it mean for something to be socially constructed. But I'll elaborate later.

Basically, when someone asks me what I mean when I say something is socially constructed, I say, "Did humans do something about it, did they create it, or do they define if it's being done in the right or wrong way? It's probably socially constructed, then."

Feel free to disagree in the comments.