Mac App Store First Time Setup Pop Up?
That's a new Privacy process just introduced by Apple.  There's a new popup window that will display when you log into your iCloud account for the first time after the upgrade.


You can read more about it here.

After installing the newly released iOS 11.3, macOS 10.13.4, and tvOS 11.3 software update, users will be met with a page that explains how a new icon, of two figures shaking hands, will appear “when an Apple feature asks to use your personal information”.
The idea is that when personal information is being collected on iTunes, the App Store or other Apple services, users can tap through the icon to read more information about what sort of data is being asked for and how it is being used. It’s pitched by the company as a way for it to increase transparency around how it handles user data, aligned with GDPR rules.