The profile notes feature on Mastodon is very useful but also perhaps slightly unclear if you've never used it before.
It works like this:
Go to a person's profile by clicking on their name, look for the "Note" section and then click on "Click to add note", start typing and it saves it automatically. The note is only visible to you.
This is really handy if you interact with lots of people and need to remember (for example) why you follow someone etc.
