A pickup line is usually an expression meant to start a conversation. Usually, "pick up" means to take a women home for sex but sometimes women pick up men.
Example: "You are the most beautiful girl in this bar. What's your name?"
(after a few drinks) ... "Come home with me. We'll have a good breakfast in the morning."
Some pickup lines are sexually suggestive and I assume are used by prostitutes to start a "business" conversation.
Example: "Is your name candle? because I wanna blow you." (this is an offer of oral sex)