Something that's uncontaminated is clean and pure — it hasn't been exposed to anything dirty or poisonous. Not everyone is lucky enough to have uncontaminated water to drink.
Whether your drinking water comes from a city system or a well, it gets tested periodically to make sure it's uncontaminated and safe for drinking. If something is polluted or poisoned, like when there's too much lead in the soil or radiation in the atmosphere, it's contaminated. Add the prefix un-, "not," and you get uncontaminated. You can also use this adjective figuratively to mean "not corrupted," like your little brother who's uncontaminated by the internet.