Basically it means that the latest automatic backup Trainz has made of your current route and/or session is different from your last manually saved route and/or session. When you make a manual save the two are synchronized so the question does not arise.
As Jordon has pointed out this is usually the result of a crash to desktop, or other reason, that can cause an open route/session to be loaded into Trainz. The automatic backups are normally set to 7 per day but this can be set to whatever you want, including 0 which means NO AUTOMATIC BACKUPS.
If you Revert, then the changes that are stored in the backup (that make it different to your manual saves) are ignored and the last manually saved changes are kept. In the TRS19 beta I think they have changed the wording of this option to make it clearer but I cannot recall what it now is (it occurs very very rarely on my systems).
If you Keep, then the changes stored in the latest backup will be loaded into Trainz and become the new current version. Perform a manual save immediately after selecting this option to synchronize the manual and automatic saves.