It means that the person is being insincere in his flattery.
Suppose you've been sick with the flu for a week. You're pale, your hair is a mess, you haven't bathed in days, and you're wearing your old, worn-out pajamas. Then your husband comes into the room and says to you, "Good morning, gorgeous! Why, you look prettier than Miss Scarlett O'Hara and Elizabeth Taylor put together!"
You would tell him, "You don't have to lay it on so thick - I already told you I'd make breakfast for you."
Sometimes we refer to such flattery as a "snowjob."
"Hey mom, did I ever tell you how much I love you? And that you're the best mom in the whole world? And that all the other kids are jealous because I've got such a great mom?"
"You can stop with the snowjob, Junior - I'm not letting you borrow my car."