When you change for example a user in your Active Directory or Azure AD, then you can force the synchronization with a Delta sync. This will find all changes since the last run and apply them to either side.
Delta sync is also used when you create a new user in your Active Directory and want to speed up synchronization to Microsoft 365 so you can assign the license for example.
To force an Azure AD Delta Sync we need to run the following cmdlet:
Start-ADSyncSyncCycle -PolicyType Delta