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The subscribeToCartStateUpdate function adds your callback function to the list of functions that will be called each time after the State object is changed. The callback should be passed as the only parameter.

subscribeToCartStateUpdate( myCallback );
<script type="module">
  import { subscribeToCartStateUpdate } from '{{ "liquid-ajax-cart-v1.11.2.js" | asset_url }}'

  subscribeToCartStateUpdate(( state, isCartUpdated ) => {
    console.log('State is updated: ', state);
    console.log('Cart state is updated: ', isCartUpdated);

Your callback will be called with the State object and the isCartUpdated boolean parameter.

If the isCartUpdated is true, it means that Liquid Ajax Cart has just received new cart JSON-data from Shopify from the last Shopify Cart Ajax API request and the new cart state is available in the cart property.

If the isCartUpdated is false, it means that only status State property is changed.