A tri-tone, obviously by it’s name is “three tones”. So 3 full steps up OR DOWN. It’s symmetrical. In terms of music theory, most people relate it to a scale. So it’s a b5 (or #4). If we take a C major scale: C D E F G A B – Our 5th note is G and we flat it. So Gb is the tritone of C.
An easy way to see it on the guitar is to play a power chord. Let’s say A. So you are playing an A on the 5th fret – 6th string, and an E on the 7th fret – 5th string. Drop your E a half step to Eb, (6th fret-5th string). A-Eb There’s your tri-tone.