RPG Elements is a catch-all term for game mechanics which are typically found in role-playing games and is used to refer to when they are implemented into a non-RPG title.
These elements are largely drawn from tabletop role-playing games (also known as pen and paper role-playing games), but also their video game counterparts. They are often used to enhance role-playing by getting the player to be tied closer to their character and adventure overall through moral choices leading to branching paths & alternative endings as well as character customization like in Metal Gear Solid V: The Phantom Pain.
Damage Numbers is a common element from RPGs which now appears in all sorts of games including fighting games. One example of a side-scrolling beat em up with the feature is Dragon Ball: Advanced Adventure.
RNG (Random Number Generation) calls back to dice which could feature 20 sides in tabletop RPGs as. It can range from affecting card draws like in Hearthstone or the random loot (also an RPG Element) gained in games. This is commonly accompanied with Luck as an individual stat which can be manipulated through equipment or leveling up. The latter of which is one of the most common elements is applying to characters and attributes in Viva Pinata.
Inventory systems as seen in the Resident Evil series and numerous adventure games, allow players to use, combine and store many items tend to be common in RPGs of all kinds as along with quests as seen in Red Dead Revolver.
The ubiquity of leveling up and some other elements often leads to confusion regarding whether some titles have substantial RPG elements or have enough to qualify as one of many RPG sub-genres.