It's kinda like "The Game", if you have to ask "What is The Game?", you already lost. And in the same manner with the patch, if you have to ask, come back at a later time, once you are wise to the ins and outs of the world of adult visual novels.
Nah, just kidding, ever since Patreon laid the smackdown on creators, devs themselves, or passionate modders make patches, that circumvent the stupidity of Patreon, and allows devs and players to enjoy the games as they were intended, in their original "family friendly" form. In other words, enabling, and sometimes even adding (incest) content, so players can avoid the curse of "The Landlady" and "Tennants/Roommates", making the characters in question, family instead of strangers. And I'm sure someone else already told you this by now, but if they didn't, now you know. And there are other patches too, for other types of content, and patches that fixes bugs, or improves on the visuals in a game. Hope that clears things up for you.