Pending invitations are invitations that have not yet been accepted or rejected by invited collaborators. If you want to remove/re-send a pending collaboration invitation, go to the folder/file and look at the Collaborators panel on the right. Here you will see a list of all confirmed and pending collaborators.
Click the ellipses (...) beside the collaborator's email address.
This will bring up a menu with options to Resend Invite or Remove the collaborator.