Like the difference between "meaning" and "definition".
"Signifo" (meaning) is more abstract. "Difino" (definition) is a more concrete expression, laid down in written words (or spoken, I suppose), that helps you to understand the meaning of something.
You can also look for a meaning in a whole phrase, for example, or even in something non-verbal such as a certain look, but you wouldn't define them -- definitions are mostly for individual words, I would say.
So you could ask "Kion signifas 'I love you'?" but you couldn't ask someone, "Bonvole difinu 'I love you'", I think (though you could ask "Bonvole difinu 'amo'.")