What this is in reference to is the amount of display pixels on the TV's screen. 5 megapixels is nice and eight will be even nicer. Of course, it also depends on the size of the display too, in meaning the larger the screen size, the higher pixels will provide for a better picture, over all.
Just remember, the original picture source will also play a large role in how well your picture looks. Most (if not all) retailers will display the best possible picture source to their sets on display and it's very likely that when you get it home, the TV won't look quite as good as it did in the store.
Hope this helps,