What are some "essential" things with music theory that every musician should know?

Excerpt from the original

Due to a recent wrist injury I'm unable to play guitar for the next several months, so I've started taking music theory lessons with my guitar teacher. He's a fantastic teacher, but because I'm an adult he basically leaves it entirely in my hands as to what I want to learn for each lesson - he likes to give his older students the freedom to learn what they choose. It's great for when I have questions that I really want answered, but I currently feel like I'm running out of things to learn and questions to ask just because I feel like I don't *know* enough about theory to know what to ask. We've gone over the chromatic scale, circle of fifths, intervals, how chords are built, scales and modes, ear training and a few other things I'm forgetting right at this moment. I'm planning to ask him to teach me to read sheet music next because it's been bothering me for the last few years now that I don't know it. But beyond that, I'm running out of questions. Sometimes it feels like he's **too** efficient and answers multiple questions I have in one lesson (without me having to ask more than one of them) which is great but it also leaves me with less to ask the next time around. My knowled