Ask HN: What's pro-rata? (And How to learn that related stuff?)

With the recent top post about YC "adding a pro rata provision" I feel like a complete n00b in these topics. I'm a startup founder and we're profitable, but I'm not familiar with all these terms. Maybe because we've bootstrapped and never need to raise money, but I still want to learn in case we ever receive private funding.

Related question: I remember reading a post a woman wrote about how a founder can get screwed and lose the all the rights to his/her company when the company was sold. Anyone remembers?

Sorry for the extra-n00b. Thanks!