Non-Uniform Scaling during UV unwrap. What is this? I've got a problem while unwrapping UVs. When I'm unwrapping UVs it showing me this message.   

Why is this happening?
It's because your object has been scaled on some axes more than others in object mode.

For example:



It should be fine, but if you are having problems you could try applying the scale by pressing CtrlA> Scale in object mode. This will apply the object mode scale "offsets" to the mesh data, resetting the offsets to 1 without  changing the appearance of the model. Note that this could affect modifiers such as bevel. See Why are the longer edges being beveled more?