A bit of googling and I find that the classic line to illustrate plot is attributed to EM Forster:
The king died and then the queen died is a narrative.
The king died and then the queen died of grief is a plot.
It's about cause and effect. Actions leading to events which lead to further actions - at the end of which the people involved are changed in some way - be it physically or mentally. They fall in love, they solve the mystery, they realise that their values have changed, they reach some kind of understanding or enlightenment. All good stories - genre and literary fiction - follow this path in some way, whether the characters reach a literal or spiritual destiny.
Taking it a stage further - events need to be a direct result of a character's actions. How many times have you read stories where a character sits passively while things happen to him? Not very often, I expect. Character generates plot which in turn changes the character. It's why deus ex machina endings are so frustrating - God in the machine - where in classical literature, the Gods would walk onto the page and determine the ending. In modern fiction it's where the characters wake up and it's all been a dream. Or where the murderer is revealed to be somebody's twin brother that's never been mentioned in the story. As a reader you feel cheated and justifiably so. Events have to be connected - cause and effect.
In a short story, it's the difference - for me, anyway - between a story and a piece of prose. A piece of prose is words on a page or screen, in the right order. It may be beautifully written with wonderful imagery - but if it doesn't end, figuratively speaking, in a different place from where it started, it's not a story. There's no change, no growth, no enlightenment in the character or even reader. That's not to say there is anything wrong with pure prose - I just don't like seeing the two confused.
I find this really hard sometimes. I'll be writing a scene and I'll suddenly realise that everything is happening around my character and he has no direct influence on any of it. With the nature of the stuff that I write, it's quite common, and yet the good guys have to escape/outwit the bad guys by virtue of something they themselves initiate. Otherwise what was the point? Calling in the cavalry is the easy option.
And while I don't always have happy endings, I do try to make them satisfying within the context of the story. But never walking off hand in hand into the sunset!