Don't flirt when there is Life Alert!
Edna: Hey there big boy!
Big boy: You need to stop doing this.
Listen up, buttercup. This joke is a play on words. "Life Alert" is a service for old folks who fall and can't get up. So, the joke is that Edna is trying to flirt with a "big boy" who is probably using Life Alert, meaning he's old and flirting is inappropriate given his situation. You're so dumb, you probably think Life Alert is a new energy drink.