All open data are good, but some are better.
In a colloquial way: open means that everyone can use it, for any purpose. The idea is that you have as few technical, financial and legal barriers as possible.
Quite a popular definition is the one from the OKF:
“A piece of data or content is open if anyone is free to use, reuse, and redistribute it — subject only, at most, to the requirement to attribute and/or share-alike.”
I believe this becomes more clear when we enter the "grey areas":
- Open usually means free. However, sometimes it may include a nominal fee when the publication of data is costly.
- Open usually avoids a non-commercial licence. Excluding commercial activities brings the uncertainty of the exact definition of "non-commercial" and its potential legal ramifications.
- Twitter is also somewhere in between. Some people argue it's "open" (e.g J. Gurin in Open Data Now) perhaps because of its public nature. However, there are serious restrictions of who and how you can use Twitter data.
Glasgow
On their site they write
Glasgow City Council data is now open by default, freely and easily
accessible to all.
Unfortunately, the link to "open by default" goes nowhere at the moment. The licence would explain what you can and cannot do with it. I would assume (and I may be wrong) that the Open Government Licence applies. It is relatively generous and easy to read.