Features of a tool that make it obvious to operate correctly.
Affordances are when the features of a tool tell you how to operate it. They are a shortcut to prevent you from having to think. You can open a door without thinking about it by looking at the jamb. You can create and utilize your own affordances to organize the world are you.
Affordances are a way to Externalizing the Brain.
If you do a bit of pre-work you can use a series of affordances as a sort of Other Forms of Checklist