Okay kiddo, let me explain what a primary triad is. It's like a special group of three notes that sound really important and are used a lot in music.
The primary triad is made up of three notes called the tonic, subdominant, and dominant. The tonic is like the home base note, it's where the melody usually ends and sounds very stable. The subdominant is like the note that takes you away from home a little, but not too far. And the dominant is like the boss of the group, it's the note that makes you want to go back to the tonic and really brings energy to the music.
So, when you hear a song that seems to have a really strong sound or feel like it's leading somewhere, it's probably using the primary triad in some way. It's kind of like a secret code that musicians use to make their music sound really good!