All overly detailed scenes, that don’t contribute to either hindering or furthering the protagonist’s quest, are unnecessary padding. All they do is clog the pace.
Unnecessary sex scenes. If the sex is not dramatic, that is, the details of the sex itself create or resolve complications, you don’t need it. If the aftermath of sex is where the drama happens, you don’t need the prior detail. Don’t try to convince modern readers that people having hot monkey sex are actually carrying on a thoughtful dramatic conversation during it: who has the brains to? You have to stop the sex cold for complicated talk. The author who tosses in sex “to keep the reader from getting bored” has not fixed but highlighted their problem: the pacing or tension has died. If it’s boring for the characters to discuss some subject with their clothes on, it’s boring plus silly for them to discuss it while their groping is graphically described. (Mind you, I can write erotica, but something besides sex is going on in each of those scenes, which makes plotting through them really difficult work. Good sex scenes aren’t easy.)
Unnecessary violence scenes. If the writer does graphic violence, not to show its internal dramatic effect on a character, just to show it, it’s so much padding. Highly detailed forensics reports are boring. I’m thinking of The Phoenix Guard, recommended to me as mannerpunk, where I bailed out in Ch.3 because the characters knew what the damages to their opponents were, as if they had x-ray vision. I think all fight scenes should be considered as if you were a good combat choreographer for stage or screen: what is the actor saying dramatically here? Only in fiction you can present the internal thoughts and feelings directly. Otherwise, it often turns into the “unnecessary expertise” scene (see below). Good books on fight arranging are your friend here, as they will point out how a fight can be a series of non-speaking dramatic events.
Unnecessary tour scenes. Luck in the Shadows comes to a dead halt, in what has been a menacing emergency, so Alec can take a stroll around the grounds of the college of wizards and see how cool and groovy it is. I quit reading that book not long after. Tours mainly belong early in a book, for locations where a lot happens, when knowing the layout is necessary to the reader. Alec didn’t have to go on a tour, since the one tower and the people who come to it are all that really matters.
Unnecessary expertise scenes or infodumps. We can call this the Herman Melville Syndrome. No one needs to know that much! It’s just the writer halting things to show off their research/world building. The Turkey City Lexicon calls it “I Did My Research and Now You’re Going to Suffer for It”: wordy but accurate. Historical fiction is terribly prone to this, as may be something set in a place or a work environment you had to research greatly. This is what happens when World Presentation goes wrong.
Unnecessary entertainment scenes. You know, all the lyrics of the whole epic song, when all that matters is the first fifteen lines (no names, but one offender’s initials are JRRT, and he inflicted two language versions). Ditto the detailed description of an intricate dance which means nothing to the plot, danced by people who will not be seen again, or that creates only predictable reactions. “He watched the girl dance and found himself slipping from lust to adoration,” is quite sufficient. A couple of paragraphs, okay. Pages, no. Labanotation® is bor-ring to read.
Of course there are necessary versions of these, but when they are necessary (love scenes in romances, combat in a/a books about warrior bands) and done dramatically, not plonkingly, readers don’t mind. Even in erotica, the experts say each sex scene should still have a secondary purpose of creating or resolving problems, even if only partially. That’s what all scenes should do.
So when you find these padding scenes in your work, you have the choice to pare them way back or eliminate them, or else give them some real purpose.