When an earlier bit of action or dialogue touches on the same ground as a more important bit of action or dialogue later on. It's like preparing the reader for what is coming. For example, in Butch Cassidy and the Sundance Kid, Ella says that she'll come with them to Bolivia, she'll cook for them, clean for them, do everything for them - but she won't watch them die. Later on, when they're in Bolivia and the posse chasing them is getting very close, Ella says she's going to return to the US. We know, because of the earlier bit of dialogue, that she thinks they're going to die.
Characters who are similar to the main character and reflect (usually less desirable) aspects of their character or situation. Think of the sisters in Pride and Prejudice. Jane is more docile, Lydia more lively than Lizzie. Friend Charlotte is more sensible. All their fates could have been Lizzie's if she'd behaved like them ie lost Bingley (it's only through Lizzie and Darcy's actions that he comes back), run off with Wickham or married Mr Collins.
Telling us information, whether through dialogue or narrative. Thrillers often have lots of exposition - for example Day of the Jackal, which told us all how to get a false passport among other useful bits of info if you were planning a career as an assassin. Exposition is clunkiest when chunks of it are dropped into dialogue. "Hello Carruthers, it's years since I've seen you. Must have been last at school, sitting next to Jenkins in Geometry. He's done very well for himself, become head of MI5 now you know, married to that girl from the grammar school - Mavis, that was her name. You were friendly with her once, weren't you?"
Is when you go back in time and the next scenes are all in the past. "It had happened when she had been at Granny's, all those years ago. Granny had been helping her with putting on her boots, when Rufus had rushed into the room, shouting that the village hall was on fire. They had all run out of Granny's cottage, and hurried down the hill...." Use sparingly.
Is not quite the same as flashback, in that it's just information about a character's past, rather than showing it in a scene. "She'd not seen such expensive dresses, not since that summer she'd worked at Harrods when she was a student." Again, use sparingly. Readers don't usually need to know much history about characters, and if they do, try to show it through their current actions and dialogue.
Protagonist and Antagonist
The protagonist is the main character, the one who makes stuff happen. The antagonist is the character who opposes them. They're terms taken from Classical Theatre, as written about by Aristotle in Poetics.
Point of View. The subject of tomorrow's post.