macos – Password required to update Apple ID settings

My iPhone (under iOS 12.1) informs me that I should "Update Apple ID settings" otherwise "Some account services will only be available [I] log in again. "

When I launch settings > Update Apple ID settings iOS asks me first my "Apple ID password", then "iPhone password", then "Mac password". The description of the latter reads as follows: "Enter the password you use to unlock the Mac …".

What password is it? I'm hesitant to enter a lot of passwords without better information on what's going on here. I have both an iPhone and a MacBook and use the same Apple ID account on both.