For most configuration needs, check out the definitions directory.

├── definitions
    ├── actions.lua
    ├── config.lua
    ├── global.lua
    ├── main.lua
    └── midi.lua

These files contain lua tables that may be altered to change or add key bindings, actions, or tweak reaper-keys behaviour.

Read on to learn how to configure a particular aspect of reaper-keys.