A client ID, also known as an application ID, is a unique identifier assigned to an application or service principal when it is registered in Azure Active Directory (AAD). The client ID is used in conjunction with the secret key to authenticate the application or service principal to Azure.
To use Azure services, an application or service principal must be registered in AAD and must have a client ID and secret key. These credentials are used to obtain an access token, which is then used to authenticate and authorise requests to resources like SharePoint etc.