In video game design, an "asset" is literally anything that can go into the game. This includes forward-facing things that users can see, such as 3D models of characters, objects, and environments, or the GUI, or things users can hear like music and sound effects, but also includes things only the game developer will notice, such as code bits like AI scripting and physics.
It is a very common practice for people to sell assets for others to use in making games (such as at the Unity store here, which sells assets for the Unity game engine). When you purchase the assets, you're buying the right to use them in your game, and ownership of the asset remains with the original creator. The purpose of these assets are for game developers to build on top of those assets; you may purchase a 3D character model, but ideally you'd put your own skin over it and change it to stylistically be part of the world you are creating, as opposed to the generic original asset.
However, lazy or unscrupulous developers will throw a bunch of premade assets into their game and call it a day. These kinds of low-effort, unoriginal games are called "Asset Flips," as they are simply buying an asset and sending it back out into the world.
The term was coined by Youtube game-reviewer Jim Sterling in this video where he deconstructs the practice in relation to the deluge of asset flips in the Steam game store.