What is access token?

Authentication is the process of verifying the identity of the user or the app. To keep transactions on Shopify’s platform safe and secure, all apps connecting with Shopify APIs must authenticate when making API requests.

Authorization is the process of giving permissions to apps. When an app user installs a Shopify app they authorize the app, enabling the app to acquire an access token. For example, an app might be authorized to access orders and product data in a store.

Shopify document: https://shopify.dev/docs/apps/build/authentication-authorization