As explained elsewhere, every story is: the protagonist (see below) wants something and can't have it. The antagonist is the reason he or she can't have what they want. It can be another person or it can also be a force of nature - e.g. if the protagonist is caught on a mountain in a blizzard, and wants to get to safety, the antagonist is the blizzard.
Or it can be a set of circumstances - e.g. the war which keeps the lovers apart.
A beta reader is somebody you know who will read your finished manuscript and critique it for you before you send it to agents/publishers. It could be a friend or family-member, or a member of your writing group.
The word Denouement comes from the French and literally means 'unknotting'. It's the part of the story which takes place after the story climax. It involves the unravelling, and sorting out, of all the subplots and also a brief period of enjoying the successful outcome of the story.
In medias res
In medias res is Latin for in the middle of things and it's a phrase often used to describe the decision to start your story as dramatic action is happening, rather than with description or setting. It's usually used as a narrative hook.
Plot-holes are places in the plot where inconsistencies arise. It can be as simple as the heroine having brown eyes to start with and later having blue eyes. Or it can be as major as the hero suddenly being able to defuse bombs even though no prior hint was ever given that he could do this.
The protagonist is quite simply the main character, the hero of the story (and both men and women can be heroes in this context).
The slushpile (or slush pile) is the name given to the pile of unsolicited manuscripts received by agents and publishers.