The title sentence is a dialogue from "Friends" (the sitcom):
Monica has become acquainted with a man named Paul. On a date Paul says to Monica: "Since my girlfriend left me I haven't had sex with any girls for two years" (of course he was lying for attracting Monica's pity) and Monica believes him and pities him. And then one night she invites him to her home and ... Then when Monica finds that he is a liar, she tells the story to her friends. Joey (Monica's friend) says laughingly: "Of course, it was a line".
In the "a line", is "a" an article or something else? And what meaning does "it was a line" have?