A "pickup line" is something that you say to people who you're attracted to, to start a conversation with them. Pickup lines can be simple, like this:
Hi, what's your name? I'm Patrick.
There are also a wide variety of clever or funny pickup lines that people make up, like:
Heaven must be missing an angel, because here's one right here on Earth.
When someone uses a pre-written pickup line or repeats the same introduction to multiple people, you can call that "using a line":
He used some cheesy pickup line on me. I just rolled my eyes and turned the other way.