After reading roughly 10,000 articles and books about the analysis of timbre, I can say with confidence this is how all of them start out. So here’s my own explanation of timbre’s DEAL. Timbre is more colloquially known as “tone color.” Imagine two different instruments, e.g., a violin and a trumpet, playing the same exact note at the same exact pitch, the same exact volume, and the same exact duration. You can still tell them apart, because the instruments have different timbres. You don’t need to have special training to tell that they are different; timbre is something that we intuitively understand.
In terms of how timbre relates to music, or specifically to popular music, it’s what gives each band their “sound.” It’s often said by music theorists that timbre is one of the most important aspects of popular music (e.g., Tagg 1982), while in classical music it’s maybe not so important. Even though this is generally agreed upon, music theorists still focus on things they focused on when dealing with classical music: pitch, rhythm, harmony, form.
In other words: even though timbre is highly intuitive, and so central to our experience of music, music theorists still don’t really talk about it! It’s my assertion that this is just because there is not a clear methodology that’s been established for the analysis of pitch, at least not one which is as accessible as theories of pitch/rhythm/form. I want to try and fill this gap with my own work.