On the activity page, users can write down a random thought, what they're doing, or whatever they choose. It can also be used to give shoutouts to other users. You're notified if you get mentioned in an activity, causing some users to try and communicate with friends using this feature. Many users call activities RA's (short for "Recent Activity").
The activity page is also called "the feed" by many users. They may call it "fast" if a lot of users are currently posting, or "slow" if only one or two people are posting.
Roleplay Accounts (sometimes shorthanded to RP Accounts) will usually roleplay on the feed as well.