The UE40JU6400 has a maximum display resolution of 4K Ultra HD. That's over four times the display resolution of the most common HD type of Full HD. 4K Ultra HD TVs have a resolution of 3840 x 2160 pixels.
A 4K Ultra HD HD type is actually the best display resolution across all mid-size TVs that have been available for around the same time as the UE40JU6400.
An HD Ready HD type is the worst display resolution across similar types of television.
4K Ultra HD TVs have a resolution of 3840 x 2160 pixels.
Alternative 4K Ultra HD TVs currently available which you might also want to consider include the LG 40UF671 which is cheaper.