xmodmap is a program that allows you to modify the key mapping in an X Window System session. The challenge is that many keyboard layouts exist around the globe. When a key is pressed, the keyboard transmits the key's scancode, that is, the key's position. It does not transmit which key was actually pressed, the operating system has to do the translation scancode/character. E.g. what is a "y" on a US keyboard is a "z" on a german keyboard. See background: How keyboards work.
With xmodmap you can modify the scancode/character mapping.