What does the phrase "tell us what you really think" mean?

Solution 1:

The phrase is often used ironically as a reaction to a strongly-expressed opinion.

A non-ironic use, to draw out a stronger opinion:

"So, what do you think of the website's new layout?"

"Oh, it's okay..."

"Come on, even I know there are a few mistakes in there; tell us what you really think."

An ironic use, as a reaction to a strong opinion:

"I hate him, I never want to see him again, and I hope he catches a terrible disease!"

"But tell us what you really think about him, Sally."

Solution 2:

It might be used when it is apparent that speakers are not expressing their true feelings. Tell us what you really think is an encouragment to them be honest about them.