Are just pixels that do not respond. They will usually be black, and verrrrrrrrry small. If you put up a solid color screen, any light color, you may be able to see one, it'll be just a little black dot. Under normal viewing you'll probably never see one. Picture this: paint an entire wall white. Now take a needle and put a hole anywhere. Step back about fifteen feet and see if you can see the pin hole.
But note this. No manufacturer will replace a screen with only one dead pixel. The minimum number is usually eight, and only if they are in a cluster, and then don't count on it. You'll need to look at the warranty on the screen for the details of dead pixels.