What does stand-alone mean?

Hi all. I've been told that it's very hard to publish a first novel that is not a stand-alone. My problem is that I'm not sure exactly where the line between a stand-alone and a book in a series that doesn't stand alone.

I've written a book that is planned as the first book of a trilogy. It ends with the protagonists friend betraying her and turning into the primary antagonist. The antagonist escapes and there's a build up towards the protagonist hunting her down in Book 2.

It doesn't, however, end on a cliffhanger or anything like that - and a lot of the plot threads are tied up. I've tried to make it a satisfying ending. The protagonist has a very definite character arc that she completes by the end.

So is it possible to call that type of novel a stand-alone?

Thank you very much for all the responses. I'm still not 100% sure whether I could get away with calling it standalone. But I guess agents can make their own mind up based on the synopsis.

'Basically it should be a book in its own right ( with its own arc and resolution) - only those little, tiny ambiguous threads you left kinda out there - they are so coming into the next book.'

Yes, that's just it. While I think it has its own arc and resolution, I don't think my ambiguos threads left over are 'tiny'. I think they're rather large. As a reader, a rather large part of one's mind would be thinking, 'that's great, but what happens next.' Which if I understand everyone right, means it's not standalone.

Cheers for everyone's thoughts!