An adverb meaning 'probably' or 'likely'; expresses the speaker's expectation that something is likely to happen.
多分明日は雨が降るでしょう。
It will probably rain tomorrow.
彼は仕事で忙しいから、多分来られないと思う。
He's busy with work, so I think he probably can't come.
多分そうだと思うが、確認してみるね。
I think that's probably the case, but I'll check.
An older noun/adjectival use meaning 'many' or 'a large amount' (often in the form '多分の...'); less common in modern speech, where 多くの is usually preferred.
その計画には多分の費用がかかると見積もられている。
The plan is estimated to require a considerable amount of expenses.
多分の時間を割いて準備した甲斐があった。
It was worth taking a considerable amount of time to prepare.