Once a 3d model of any kind has been created, the next step is often to start adding textures to it. However, there is a problem – You can’t just apply something 2 dimensional to a 3 dimensional object. Although there are methods to do so that are not recommended for various reasons, that’s where unwrapping comes in.
Unwrapping is the method used to flatten out a 3d model into something 2 dimensional – making it a lot easier and a lot more efficient to add textures to it. This allows mere humans to apply a 2d texture to the faces of a 3d object via a 2d ‘map’. The process is much like taking a cardboard box, cutting the edges and flattening down neatly. The keyword there is neatly – It has to be done neatly or else you will either have no clue when adding textures, a hard time adding textures, or it will be impossible to add them altogether because they are a clustered mess.
Another way of thinking about it are geometry nets you can download off the web, cut out and sick together to form a 3d shape. It’s the exact same idea, just in reverse.
Since the X,Y,Z axis are already in use by 3d space, the axis used for UVs or ‘Texture Space’ are referred to as U and V, which is where it gets its name from.
Unwrapping isn’t as simple as just unwrapping everything and putting it into the texture space. That texture space is all you have, so you will need to prioritise parts of your mesh and overlap others to make the most of whats available. If you have multiple parts of a scene made of the same material – wood for example and don’t need any variation between them, you can place them all in the same place on your texture map and they will all share the same texture. This will save you a lot of space for things which are more important.