It really doesn't.
The Dneiper lies between the lands russia occupies and the Moldovian border, and requires going through Kherson and Odessa to get there. The problem is that russia no longer has the means to cross the Dneiper again after retreating across it. The bridges are gone and russia has proven inept at river crossings, even ones much, much smaller than the Dneiper.
The best russia can do with its poorly trained, poorly equipped hordes is take a small amount of ground for terrible losses. They are not going to get near Moldova.