Veronica8 Basically refers to a situation where everyone trash-talks the main character or insults him/her, often thinking that he/she is trash or low status or whatever. Then it turns out that the main character is either super-rich, or has a powerful backer behind him/her, or is super-powerful and defeats the so-called strongest genius, or use politics to outsmart his/her attackers. Then all the people calling him/her trash or insulting him/her as having low status/no background and connections/poor/weak, etc. end up being humiliated because they trash-talked the main character so loudly earlier, and he/she proved them wrong to such a huge extent that it feels like a slap in the face. You know, like how your face burns when you're super-embarrassed?
Can also be a literal slapping, where the villain insults or tries to get the main character into trouble, but because the main character ends up being strong/has powerful backing or connections, etc. he/she can slap the villain for insulting him/her and there's nothing the poor bugger can do about it.
The purpose of the trope is usually wish fulfilment or power fantasy, especially for people who are often scorned upon or looked down in life, only to reveal that they are actually powerful and can slap other people for looking down on them, then get away with it. Sort of like the time when you faced bullying during your childhood, and you resent people for insulting you/looking down on you. So you want that feeling of elation when you take revenge on them and prove them/their insults wrong.