Upres = 'up' the 'resolution'. They're enlarging the 1280 x 720 pixels (dots that make up the picture) to fill a 1920 x 1080 pixel display. Generally, when you start enlarging pixels the picture turns to crap. They use sophisticated hardware/algorithms to maintain picture quality when it's blown up or uprezzed from a lower spatial resolution to higher resolution.
Sometimes you'll see a broadcaster not upres a TV commercial and it appears on your HD TV in a smaller box.