From Autodesk's Performance document mentioned in THIS POST:
When attempts to Save to Central (STC) collide, Revit-based applications display a dialog notifying that another user is currently saving to central. Cancelling Save to Central will prevent queuing the save request, allowing the user to continue to edit the local file before another Save to Central command.
Written another way... You can either leave the message alone until Revit returns to deal with your request or you can choose to cancel your STC. I usually leave the message open and wait for Revit to "get back to me". Their suggestions is to choose Cancel which allows me/you to continue working, the other user's STC will complete sooner/quicker and then we can use STC which in turn will complete faster.