Those are archaic English. They mean 'are' and 'your'.

Archaic: "Our father, which art in heaven, hallowed be thy name." Modern: "Our father, who is in heaven, holy is your name."

Those are archaic (older) forms which are not used in everyday speech anymore. So, "Our Father who art in heaven, hallowed be thy name" "art" = "are" "thy" = "your" "Our Father who are in heaven" sounds awkward, so modern translations usually phrase it something like "Our Father in heaven" or "Our Heavenly Father"