This area of the Unity User Manual provides a general overview of the principles of working with Assets in Unity.
An Asset is a representation of any item you can use in your Project. An Asset might come from a file created outside of Unity, such as a 3D Model, an audio file, or an image. There are also some Asset types that you can create in Unity, such as a ProBuilder Mesh, an Animator Controller, an Audio Mixer, or a Render Texture.