Azure AD Connect is an application responsible for synchronizing Active Directory with Azure AD allowing for a natural population of users, groups, and devices in Office 365. While for most companies standard setup is very easy and most of the time touch-free, there are companies which require greater customization. During installation of AD Connector, you choose what should be used for Azure AD Username from your AD. UserPrincipalName field is an obvious choice for this and also proposed by default for that purpose. This field is utilized further by your users to log in to your Exchange, SharePoint, Teams and so on.
