This is what the tag wiki excerpt says:
A tag for questions that may not necessarily have a clear-cut right or wrong answer and are often subjective. If it's not a bug or feature-request [sic], it is probably a discussion.
So it's helpful to frame it in the context of bugs and feature requests.
Let's say you have a problem with feature on the site and it's clearly not supposed to work like that. There's an intended functionality, and it simply isn't doing what it's intended to do. That's a bug.
Now let's say there's something you'd like to see differently about a feature on the site, different from it's current intended functionality. You have a specific change in mind and you want to see it implemented. That's a feature request.
Finally, let's say there's something that bothers you about a feature on the site, but you're not sure what the solution should be. You want to point out the problem and solicit input from other community members on:
- whether it really is a problem, and/or
- if it is a problem, what a good solution might be for it
That's a discussion.
Now, as to why there's a "not constructive" close reason even though there's a discussion tag: some "questions" are not actionable: the user wants to rant about a problem, they want to joke about something, etc. They might be tangentially about the site, but they really aren't contributing to the betterment of it. That's not constructive. Different from a productive discussion about how to fix a problem with the site.
The "not constructive" close reason could be made clearer, but the close reason is global across all sites on the network, and this is honestly the first time I've seen a complaint about it: if you're using a question to discuss an aspect of the site in a productive manner, there's no chance of it getting closed as not constructive.