My iPhone (under iOS 12.1) informs me that I should "Update Apple ID Settings" for otherwise "Some account services will not be available until [I] sign in again."
When I launch
Update Apple ID Settings iOS first promts me for my "Apple ID Password", then "iPhone Passcode", and then "Mac Password". The description for the last one reads: "Enter the password you use to unlock the Mac ...".
What password does this refer to? I hesitate to type in lots of passwords without better information on what's going on here. I do own both an iPhone and a MacBook and use the same Apple ID account on both.