Definition
To speak frankly, to convey all and only the truth of a situation.
How People Actually Use It
To state what you believe to be true without forethought. Usually, people who "tell it like it is" hate it when other people "tell it like it is" about them.
"Trump: The FAKE NEWS media doesn't like me because I tell it like it is!!! Bannon: The meeting at Trump Tower with Russians for dirt on Hillary was treasonous. Trump: WAH, WHY DID YOU HAVE TO TELL IT LIKE IT IS???"
Source: Urban Dictionary