How Do I Access Parameters Emitted by an Action?

If you look at the documentation for add_action, you see that it accepts 4 parameters. The first 2 are required for adding any actions. If you want to change the priority of your action, you use the third parameter. If you want to change the number of arguments the function accepts, you use the fourth parameter.

For your example, the number of arguments is 1, so we use 1 as the fourth parameter. We still need the the third parameter though, so we use the default value of 10.

add_action( 'woocommerce_subscription_payment_complete', 'wpse_106269_payment_complete', 10, 1 );
function wpse_106269_payment_complete( $subscription ) {
  //* Use $subscription
}