斗胆 is a polite, self-deprecating expression meaning 'to take the liberty of' or 'to be so bold as to' — you use it before asking a question or making a request that might be presumptuous, especially in formal or respectful contexts. It literally means 'to contend with courage' but functions as a humble adverbial phrase, often followed by verbs like 问 (ask), 说 (say), or 建议 (suggest).