The verb boire (meaning to drink) is an irregular verb, which means that its forms have to be learned by heart because you can't just make them following some rules. In the case of this verb, its past participle is D. bu.
Past participle is used to form a couple of complex tenses in French, such as passé composé, plus-que-parfait, passé antérieur, futur antérieur, and tenses in the conditional and subjunctive forms.
