The gods are laughing at their little joke on musicians. When it comes to the tuning of instruments, especially keyboards and fretted instruments, nature drops a giant hairball in our path. Here's a short course on the arcana of tuning. It will take us to the meaning of a celebrated collection of keyboard pieces: J. S. Bach's The Well-Tempered Clavier, humankind's greatest musical riposte to the laughter of the gods.
In dealing with tuning, there are two main terms to know. One is interval. It means the distance between notes. The basic science of intervals was laid out in ancient Greece, perhaps first by the mathematician Pythagoras. The first notes of the C major scale are C, D, E, F, and G. The note E is the third note up from C, so the interval C-E is a third. The note G is five notes up, so C-G is a fifth. So musical intervals run second, third, fourth, fifth, and so on. (Some intervals can be major, like F to A, or minor, like F to A flat.)