It's actually not that serious. Version blocking is implemented when a program update addresses a calculation or e-file schema (code formatting) that prevents an incorrect filing and or e-file rejection from occurring.
If you attempt to transmit a return when version blocking has been implemented, the system will not allow you to transmit the return until running the program update.
As usual, give your returns a second review for accuracy after running the update and before transmitting your return.
Good point @strongsilence if you're in the Hosted environment, the updates go out overnight to prevent interrupting your normal workday flow. If you get the version blocking message in the Hosted environment, you'll want to attempt the next morning.