configuration.nix

{ config, lib, ... }:
{
  imports = [ ./example-module.nix ];
  
  example.enable = true;
}

example-module.nix

{ config, lib, ... }:
with lib;
{
  options = {
    example.enable = mkOption {
      type = types.bool;
      default = false;
    };
  };

  config = mkIf config.example.enable {
    # stuff...
  };
}