Azure Functions lets you access your function endpoints in different authorization scopes like System level, admin level, user level and anonymous level.
While function keys can help prevent unauthorized access, the only way to completely secure your function endpoints is to use affirmative authentication for all clients who visit your functions. You can then make authorization decisions based on identity.
By providing out-of-the-box authentication with federated identity providers, the built-in authentication functionality for App Service and Azure Functions can save you time and effort, allowing you to focus on the rest of your application.

When you enable authentication and authorization with one of these providers, its sign-in endpoint is available for user authentication and for validation of authentication tokens from the provider. You can provide your users with any number of these sign-in options.
Azure Functions uses Azure Active Directory (Azure AD) as the default identity and access management service.
Please refer the what are the built-in authentications provided and customization details, authorization scope of azure functions and azure function security system in base line for detailed information.