What does it mean when a camera is being sold as body only?

I'm looking to buy a camera as a gift. I will likely go with a used camera. Someone is selling a Nikon D7000 listed as "body only." What does this mean and does it make the value of the camera go down?
Also, what are some other things I should know about used cameras that may depreciate their value or increase it?

I'm not very familiar with the camera market so please use as simple terms as possible.

It does not come with a lens.

The d7000 is an SLR, and it uses interchangeable lenses. So it can be sold with or without lenses. Some people (like me) buy the body only because we already have lenses from a previous camera that will work with the new one, or we do not think the lens that is packaged with the camera is high quality, so we want to buy a better one and not waste the money on something we will never use.