A Tritone is a Sharp Fourth (#4) or Flat Fifth (b5) of a scale. Commonly used in music to create a smooth/cool sound (As in the blues.) or to create a brooding and/or mysterious atmosphere (Common in rock and metal playing.).
A good example of Tritone use can be found in the Black Sabbath song "Black Sabbath.". The Black Sabbath guitarist, Tony Iommi is known for pionereing of the tritone to create heavy riffs. He learned to apply this after playing a series of notes, creating a tune that had his "Hair standing on end.".
The opening of the Jimi Hendrix song "Purple Haze" starts with the guitar playing alternating Bb pitches, while the bassist plays alternating E pitches. The Bb is a tritone of of the E scale the song is played in.
To find a tritone on your guitar, select a fret and move 3 whole steps (That's six half-steps, or six frets away from the note you chose,) down the neck. For example: If I selected the A at the 5th fret of the E string, I would move 3 whole steps down to the 11th fret of the E string. The note would be Eb.
An easier reference point for the tritone is to move down one string and over one fret. For example, start back at the 5th fret A on the E string. Move you finger onto the 5th fret of the A sting and move down (Toward the body of the guitar.) one fret. You will find an Eb on the 6th fret of the A string. Remember it's a flat 5th, so just remember the form of a basic power chord (1 - 5) and move the non-root note back a fret.