A one-shot, as I understand it, is the equivalent of a short story in writing. Basically it's a complete story that's told in a short number of pages. Some people use them in a similar way to a pilot episode to a TV series (where if it does enough, they'll either keep going from where the story ended, or even just start over with a proper "chapter 1" for that story), and other ones are just standalone short stories, not intended to be expanded upon regardless.
For example the comic I'm currently drawing and uploading (Shameless plug~ xD) is a one-shot that's intended to be +/- 60 pages as well (I think it'll end up being closer to 70, tbh). But I've already decided that I'm enjoying it enough that I'm going to continue it as an episodic series for a little while after.
I definitely feel like I was a little over-ambitious with the amount of content I tried to pack into this many pages though, so my pacing feels a bit rushed overall :X If you'd like to see the storyboard for it feel free to shoot me a PM and I can show you~ If you wanna get a feel for what something this length might look like*
*by a total comic newbie, granted.