Inhibits xanthine oxidase, conversion of xanthine to uric acid. Also used in lymphoma and leukemia to prevent tumor lysis–associated urate nephropathy. Increased concentrations of azathioprine and 6-MP (both normally metabolized by xanthine oxidase).
Yeah it's just saying is that the concentrations of 6-MP and azathio in the body increase when you combine them with allopurinol because they aren't broken down by XO. It's just a classic FA fragment sentence. You would decrease doses in this case.