More than just the truth, the double truth is something more important than just "the truth". Usually a bigger meaning. Something that is very very profound would be the triple truth, but that is very rare.
Made popular by Samuel L. Jackson as Mr. Sr. Love Daddy in Spike Lee's "Do the Right Thing"
"Yo! Time out! Hold up! Y'all take a chill, you need to cool that sh!t out!" And that's the double truth, RUTH!"