The .NET Secret Manager is a tool that helps developers protect sensitive information such as passwords and API keys from being stored in source control, making it easier to keep their projects secure. By using the Secret Manager, developers can save sensitive details in environment variables on their machine, rather than hardcoding them into their code. The tutorial provides step-by-step instructions on how to add the Secret Manager to an ASP.NET Core web API project and use it to protect passwords and other sensitive information. The example project demonstrates how to use the Secret Manager to connect to a Spotify API using client ID and secret credentials, making it possible for developers to build secure projects without having to manually manage sensitive information.