I just went into User CP and it said something about a userpage. What is it and what do you do with it?
In the bar at the top, there is a new option called "User Pages". Basically, this is the VGChat Encyclopedia for the masses. When you make a user page, it will automatically be posted, and other people can respond and even rank your user page.
What kind of thing do I put in it?
It's no different then a normal post. Usually, it's a description about yourself, but it's your choice (usually) what you can put in and keep out.