Qadosh is the Hebrew word for holy, and it describes first and foremost the character of God. Its root idea is set-apartness — God is other, distinct, in a category all his own, utterly pure and above all he has made. The seraphim in Isaiah's vision cry it three times over, the only attribute of God so magnified: Qadosh, qadosh, qadosh.
Because God is qadosh, everything that draws near to him must be set apart too — holy ground, holy days, a holy people. Be ye holy, for I am holy, God tells Israel. The holiness of God is what makes his grace so astonishing: the utterly pure One draws the unclean near and makes them his own. Holiness is not coldness but the blazing purity of perfect love.
Qadosh in Scripture
And one cried unto another, and said, Holy, holy, holy, is the LORD of hosts: the whole earth is full of his glory.