Learning to play in all 12 keys is an extremely valuable practice. The term "key" refers to the major scale you are playing in. We have a total of 12 musical notes (C, D, E, F, G, A, B, plus five flats/sharps in between). Each one of the 12 notes is the first note of a major scale, and each scale is a key. This gives us 12 major keys. For example, the scale of C major starts on note C. Any music that uses this scale is in the key of C major.

If you can count to 8, then you can learn to play in all 12 keys. All major keys follow the same pattern of whole and half steps between 8 notes. These 8 notes are looked at in terms of numbers. Each melody note is given a number that works in all 12 keys.

https://youtu.be/oSS9cp8Dbe8