To be "pure" in the Bible means -- unsurprisingly -- to be devoid of evil. Thus all the washing for purification prescribed in the Laws of Moses was not just a stab at public hygiene, but was symbolic of the state we should try to achieve when we approach the Lord, setting aside our evil desires and focusing on the good we want to attain. In some ways most of us still reflect this symbolism in the fact that we wash ourselves and wear nice, clean clothes to church, purity on the outside hopefully reflecting a degree of purity on the inside. Other uses of "pure" in the Bible are closely connected with a specific material: "pure gold" is common; "pure frankincense" and "pure linen" are among other uses. In these cases "pure" indicates that the spiritual quality being described -- the quality symbolized by the gold, or the frankincense, or the linen, etc. -- is genuine and authentic.