务必 (wù bì) is a formal adverb meaning 'must, be sure to' — used in written notices, instructions, or polite but firm requests. It is stronger and more emphatic than 必 (bì) and similar in strength to 非得 (fēi děi), but 非得 is more colloquial and often appears in the pattern 非得...不可. 得 (děi) is everyday spoken 'have to'.