Any piece of content in an Unreal Engine project is an Asset. You can think of Assets like building blocks that you use to create your game and application.
Assets can be of many different types, such as Static Meshes, Materials, particle systems, and sound cues. Some Assets are created outside Unreal Engine (for example, in other 3D applications like Maya or 3ds Max). Other Assets, such as Blueprints, are created directly inside the engine.
You should always manage Assets through the Content Browser.
To learn how to work with Assets in Unreal Engine, refer to the pages below.