BU stands for Bitterness Units and GU stands for Gravity Units.
By dividing your bitterness units by your gravity units you arrive at a ratio which lets you know how bitter your beer is.
A light American lager might have a ratio of 0.29 whereas an Imperial IPA might have a ratio of ~ 1.13. To view BU/GU by style, see this chart.
BU/GU is a good indication of bitterness but water chemistry, yeast selection and how you use your hops will all affect ‘perceived bitterness’.