In western music, which uses 12 chromatic pitches, a tritone is made up of 3 whole tones or 6 semitones. It’s also called an augmented 4th when the letter names are 4 apart ( ie: from C to F) or a diminished 5th when the letter names are 5 apart. When played melodically (one after another) it sounds like “Maria” from West Side Story, and when played harmonically, it supposedly sounds like the devil or something.
Because the 4th and 5th are considered “perfect”, I guess it was really awful to imagine there was a note in between them. For that reason, and because it sounds comparatively awkward, the tritone is often avoided in western music. Granted, dominant 7th and 9th chords use a tritone in their construction, but it’s not as obvious. However, 7th and 9th chords are super dissonant, and that makes them awesome in my book.
As you can imagine, this all started because the church hates satan and things that are reminiscent of him. So even though it doesn’t really sound like the devil or whatever, once composers started using the tritone, they used it to suggest evil or hell, or other such terrible stuff.
I have always found that associations with words like “evil” or even “happy” or “sad” when applied to intervals is really weird. There’s nothing inherently happy about consonance, like a perfect 5th, nor sad about a minor 3rd or something. I think it’s all about the context. So to me, a tritone really doesn’t sound evil. It is all about the tension though, and everybody loves a good resolution, right? But because it’s rarely used, it makes it super cool, something rare but not TOO weird, it definitely gets tossed around as the “devil in music” still. Although I super-duper love the Beatles, my favorite use of the tritone is YYZ by the ever awesome Rush. Does anyone really think Rush or the Beatles are evil? Pfft.