settings is a mandatory argument for any field. The value entered here will be used to store the settings in the database.
If for example you create a field using something like this:
Kirki::add_field( 'global', [
'type' => 'color',
'settings' => 'body_background_color',
'label' => esc_html__( 'This is the label', 'kirki' ),
'section' => 'my_section',
'default' => '#0088CC',
'priority' => 10,
] );then in your theme you can access that value using:
<?php $color = get_theme_mod( 'body_background_color', '#0088CC' ); ?>