This is the literal, genealogical meaning. Your 叔叔 is your father's biological younger brother. His wife would be your `婶婶 (shěnshen)`. `我的叔叔是一名医生。` `Wǒ de shūshu shì yī míng yīshēng.` `My uncle (father's younger brother) is a doctor.`
This is the most common usage a learner will encounter. It's used to address a male stranger who is roughly a generation older than you (i.e., your parents' age).
叔叔 is generally informal and warm, but carries a strong sense of respect. It is not used in formal business settings, where `先生 (xiānsheng)` is the correct term. For example, you would call your friend's dad `王叔叔 (Wáng shūshu)`, but in a business meeting, you would address him as `王先生 (Wáng xiānsheng)`.