Off-grid container buildings are ones that are relatively self-sustaining and don't require some outside utilities. Originally referring to the 'electrical grid', meaning electrical power generated and transmitted by your local electrical service companies.
In many cases, off-grid also refers to homes that don't have gas, piped drinking water, or piped sewer connections either. In short, an off-grid building is one that requires minimal if any connection to the outside world.
In some cases, this is done as a matter of principle for people who want to live a more sustainable lifestyle. In most cases though, it's done as a matter of cost and accessibility: many rural locations have inadequate access to these shared utilities, and having them brought to the project site from far away is extremely cost-prohibitive.