PROLOGUE
Hello everyone 
Welcome to my first and small yet (hopefully) effective tutorial about UV Unwrapping. This tutorial should give you a better image of how UV unwrapping works universally including some explanations on terms and what-not.
CONTENT
UV unwrapping is something you could compare to a DIY box.
For instance, you’d like to draw on a box without destroying it with pressure. Due to it breaking if you’d push on it, you have to find another way to do it.
The best and currently most efficient way to do it is by grabbing the scissor and cut open the edges until you can unwrap it as a flat object.
The exact same method is used on digital models.
U(horizontal) V(vertical) are the names of the axes of the plane that you see in the UV Editor of your 3D modelling software, due to XYZ already being taken by the 3D space’s coordinates.
The UV is a 2D representation of the model. When you export the UV from your 3D software and import it into a 2D software like Photoshop and would draw on top of the layout (creating a new layer is recommended!), you can see the same drawing back on the 3D model when you paste the texture on it at the corresponding faces.
How unwrapping would look like:
(Src: https://blender.stackexchange.com/questions/38651/what-does-unwrapping-a-model-do-and-why-is-it-important)
UV layout 2D:
(Src: https://blender.stackexchange.com/questions/38651/what-does-unwrapping-a-model-do-and-why-is-it-important)
UV back on the mesh:
(Src: https://blender.stackexchange.com/questions/38651/what-does-unwrapping-a-model-do-and-why-is-it-important)
It’s not always a box, however, hence you should learn yourself to put the seams (the cut edges) at spots you can’t or can barely see them. Because on seams there’s a hard or sharp blend, meaning that it doesn’t seamlessly blend from one side to the other. Seamless, by the way, is a word that describes something that does not have any ‘cut-open edges’ per say, meaning that the texture repeats itself forever without seeing any hard edges. While this would be ideal on a 3D model, actual seamless things are often seen on 2D textures only.
CONCLUSION
Hopefully this tutorial was helpful. 
In case you have any questions or feedback, feel free to contact me!