This idiom or expression is mainly used in American English. As you may have already guessed, a fifth wheel is a person or thing that is unnecessary or unwanted. A vehicle needs just four wheels to run smoothly. Obviously, the fifth one is an unwanted extra. The expression third wheel also means the same.
• I don’t understand why they don’t send John home when we already have three good strikers. He is a fifth wheel. (= He is unwanted.)
• The poor woman is treated like a fifth wheel by her son and daughter-in-law.
• I don’t want to work with them. They treat me like a fifth wheel.
• Peter is keen on joining the team, but really, he is a fifth wheel.
• He left the team because he felt like a fifth wheel