To me tight is that sound that you get when you play a chord and all the notes run together. You can't hear the individual notes and power chords have a sharp percussive sound to them. The tone does not feel like for one second it will break. I don't generally associate this sound with having a lot of bass, although it can have a lot of bass.
Listen to the rhythm tone on "And Justice For All".
That's my own interpretation of it.