In government, it means that government would make information like census data, statistics, licensing information easily and freely available for anyone to use, free of charge and any restrictions. It's a way of sparking creativity, crowd-sourcing new information, and basically spending less time and scarce resources in government trying to hide useful information the public should have anyway.
Officially, the provincial government here adopted the idea as official policy in 2014 but they have been typically very slow to put anything into action.
Case on point: an access to information request for data collected from caribou monitoring collars. The maps in the download are all stamped with a restriction that they are for the use of the original recipient only. No one bothered to black them out, which would be the easiest thing to do... if the restriction didn't still apply.
More importantly, though, the request was just for spot data shown on maps, as opposed to the actual latitude and longitude tracking information. A government genuinely committed to open data would have just dumped this stuff into the public domain in the first place, in bulk. That would have saved the expense of converting it into maps into the first place for this request, no matter how small the dollar cost actually was.
There is soooo much that begs to be fixed in the provincial government's access to information world.