If I may be so bold -- it means think of the book series as a series of connected complete stories, each with a start, middle and end, that together build an overall picture, rather than a miniseries or a tv show ending episodes on a cliffhanger.
The first book needs to have a main story that is wrapped up and feels satisfying, like you could tack a 'the end' on it and it would feel good. It can have other elements in it that lead into and are picked up in later books, like dropping in a character who becomes the main character of book 2, locations and myths that become important later, and so on, but the main story has to come to at least a solid pause.
Think of Star Wars, if you've seen those movies? A New Hope stands alone - the movie starts with a problem (get the droids to Alderaan), develops the problem (crap! Save the princess!), has a big climax (blow up the Death Star!) and resolves at a possible happy ever after (the big weapon is gone, the heroes live to fight another day).
Even if you never watch any of the other movies, it's a satisfying ending.
Empire Strikes Back is the opposite - it pauses, rather than ends. Han Solo's been captured, there've been chase scenes, but we know the big bad guys are still out there causing trouble. It's not really a stand-alone story. It picks up on a lot of threads set up int he first movie, uses the same characters, adds more... but doesn't have a really resolved ending as much as it has a chapter break. We know that there has to be more to the story than just that.
Return of the Jedi starts in media res - it doesn't have an easy beginning if you haven't seen the first two - but it ends gloriously, wrapping up most of the plot threads and foreshadowing laid in the earlier two. There is no doubt that the main characters will, from here on, live happily ever after.
To sell a first book, it needs to have a New Hope ending - not with everything fixed, but with the main storyline finished and left at a point where a sequel is not a necessity.
Does that help at all?
