Native resolution on your 4K TV
All modern TVs have a specific number of pixels, or "picture elements." These are the tiny dots (usually squares) that make up the screen. If you look really close, you can see them. To display a picture, the TV has to assign a color and brightness to every one of those pixels.
In the case of 4K TVs there are 3,840 of these pixels across, and 2,160 vertically, for a "native resolution" of 3,840x2,160.
If you send the TV 4K content, this, too, is 3,840 by 2,160. So the TV has very little work to do. For every pixel it has, there's a corresponding pixel in the content. If you choose the correct aspect ratio, the TV displays these 1:1, matching the source resolution and the native resolution without having to do any conversion.
But if you send the TV something other than 4K content, the TV must make the conversion. If it didn't, your Blu-ray (which is 1,920x1,080) would be a small box in the center of the screen, surrounded by black. Most of the pixels would go unused.
And most everything you'll watch today isn't in 4K.