Evenness
If you distilled all the content from Barista Hustle into one essential concept, you would end up with the word evenness. This word describes the best barista techniques, the best coffee harvest, and the imaginary ideal of the perfect grind morphology.
As your coffee-making advances and your sensory skills become tuned in, you will notice unevenness manifests in flavours as sourness and bitterness. Physically, this identifies most often as weakness and under-extraction.