DaCuda, I didn't see the lesson you are speaking of, but I can give you a heads up on the 12th fret. For instance - when you tune a string, we'll use the open "E" on the 6th string to make it easier, the note should be the same on the 12th fret as it is open. Pluck the 6th string and look at the note on your tuner, it should be an "E". Now go to the 12th fret and pluck the string it should still be "E". If it isn't the same then the intonation needs to be adjusted. I would leave this to a professional because a guitar can sound really bad if it isn't done right. I still let my guitar tech do this for me and I have been playing close to 40 years!
The 12th fret also has strong harmonics, Place your index finger across all 6 strings like a bar and touch the strings with your index finger very lightly and strum all the strings. This is a harmonic and these are also used for intonation. Hope this helps. John