Each attribute which refers to another object is considered as a link.
Example 1: user account has a manager. If you migrate the user account but you don't migrate the manager's account then you will get one unresolved link. If you migrate the managers account later then dirsync will resolve this link and you will have one unresolved object less.
Example 2: a user belongs to 5 groups but only 2 groups were migrated. You will get 3 unresolved links.
Example 3: group has 100 members, you migrate the group and 20 members (users). You will get 80 unresolved links.
This is a simple non-technical explanation which demonstrates what is being considered as unresolved link.
To find an explanation how Link Resolving works in QMM please review KB article SOL14269 "Synchronization of Linked Attributes with Quest Migration Manager for AD":
https://support.quest.com/migration-manager-for-ad/kb/14269
Usually there are always some unresolved links to be expected, especially during synchronization.
The information about unresolved objects in QMM is more of informative character and after reviewing it can be either ignored or additional steps are needed, eg. reported objects need to be included into the synchronization/migration scope.