To purify something is to remove dirt, chemicals, or anything else that it's contaminated with. You might decide to purify your tap water if it tastes like chlorine.
While scientists purify various chemicals during experiments, the substance that people most often purify is water. Water needs to be treated before it's safe to drink, and cities purify the water they send out to people's houses. You can also use the verb purify in a figurative way, to mean "make ritually or religiously clean or pure." This is the word's earliest meaning, from the Latin root purificare, "to make pure."