Loose ends are those things in the story that never get resolved.
If you like loose ends:
Some stories use this technique to end the story, especially in the thriller/horror genres. It’s a technique where the writer can let the reader decide what the real ending is. It’s often the twist that leaves the reader hanging.
For example, in the movie Premonition with Sandra Bullock, the story ends with the heroine being pregnant with her husband’s child (and her husband is dead, as far as we know). She hears the shower running. No one should be there, so she goes to check behind the curtain. Then she gasps. We never see what she saw. That’s where it ends. So you’re left wondering, “Who the heck is in the shower?” Given everything she’d been through in the movie, it could be her husband or someone else. You just don’t know.
A loose end can also be a, “Was the story real?” question. I can’t think of any story off the top of my head, but let’s say we go through the entire story and the character wakes up, thinking it was all a dream. But then, he sees something in his real life that reminds him of the “dream” he just had. And this leaves the reader wondering, “Was it really a dream, or did it really happen?”
If you don’t like loose ends:
Tying up loose ends can come in the book or later in the series, but somehow in someway, everything finds a satisfactory ending point so the reader isn’t left wondering, “What the heck?”
When I’m writing a story, I’ll spend time during the last couple of chapters figuring out where all of my unresolved plot points are. These can range from minor issues (such as friends needing to have a heart-to-heart talk after a misunderstanding) to major ones (such as tracking down the identity of the killer). Loose ends are anything you introduce in the story that need resolution in order for the story to feel complete.
In romance, it’s typical to tie up all loose ends. There might be a hint to another character’s story, but overall, the story can act as a standalone novel in the series. Often in fantasy series, there are loose ends up until the last book, though there can definitely be fantasy series where each novel in the series can be a standalone novel. (Pier Anthony is good in this area.)
If you decide to use a plot point (or two) that span the entire series, the trick is not making it such a huge loose end that the reader feels like she didn’t get a complete story. I don’t know how it is in other genres, but romance readers hate cliffhangers, even if the story is in serial format. Regardless of genre, though, if you have a cliffhanger, I recommend having the next book out quickly (like in a month or less) so you don’t leave the reader hanging for too long.
To avoid upsetting readers with an ongoing loose end in the series, I recommend that you make each book in the series a complete story. For example, Book 1 needs to have it’s major conflict resolved. Book 2 will have a different major conflict that will, in turn, be resolved by the end of that book. Same for Book 3, etc. If you are running an underlying conflict (aka. loose end) that is spanning through the series, it should be minor enough not to leave the reader upset if you don’t resolve it right away. But, the final book in the series needs to resolve this particular loose end.
(If this post was confusing, please let me know. I struggled with how to best explain this one.)