The goal of the UV unwrapping process is to flatten the 3D mesh in a 2D space in order to project textures (2D images) on it. To understand the process better, imagine that we are going to remove the skin of our alien in order to flatten it (I know that the metaphor is a little gory, but stay with us, there won't be any blood). If we have to do this, we would need to detach the skin by cutting along the imaginary seams and then flattening it down.

Another way to better understand UV unwrapping is to think about how clothes are made and take the steps in the reverse order. For instance, at the beginning, a shirt is a flat piece of tissue where all the seams are marked down. Then, the different cut pieces are attached together in order to form the volume of the shirt, like the sleeves. So, if we were unfolding a cloth along its seams, we will tell Blender where the seams are placed on our 3D mesh. After this, the ones that have been marked and the model that has been flattened...