A maintenance window is the time your IT company is allowed to work on your systems without asking first: install updates, restart computers, run backups (sample note).
For most small offices the right window is the night, roughly 19:00 to 07:00. Nobody is in the middle of a document, the internet line is quiet and a restart costs nobody any work.
Ask your provider to write the window into the contract, and to tell you every morning what ran in it. If they cannot, you do not really know what you are paying for.