To help understand what a resource in Azure AD/Entra is:

A resource is anything that is governed and protected by the Azure Entra (Azure Active directory) service. Usually, these resources are what your apps or services need to access. This document explains well how granular scopes work - Requesting scopes as a client app but as an example, a custom application/client that you built that would show the user a list of recently received mail messages and chat messages, the app would access the Microsoft Graph resource API (specifically, with the Mail.Read and Chat.Read permissions) to access user's email and chats. Each of these resources has a unique app id as well to allow for programmatic access while requesting for tokens. The image below gives you a flavor of the various resources available.