Most of the grocery stores have one of these machines near the veggie section. Yes, you need to weigh the veggies or fruits you want to buy in advance. Of course I didn't know about it the first time I shopped, and I ended up giving up all the veggies at the register. :-(
Luckily the machine has the self-definable pictures (except some of the fruits - in that case, I just have to go back to check the name in Czech) :) You just put what you want to weigh on the scale, press the picture button, and the price tag will be printed.