Minute physics does a great video on it: https://youtu.be/1Hqm0dYKUx4
It really does mean equal geometric steps between successive tones.
Each key is equally "out of tune" under equal temperament; e.g. C# minor and D minor are basically the same, modulo pitch.
Minute physics does a great video on it: https://youtu.be/1Hqm0dYKUx4