.Net Core introduces secret manager tool to store user secrets during development which helps developer to manage secrets and other sensitive settings during development in different space/file rather than on appsettings.json. You shouldn’t store id, secret keys, passwords or any other sensitive data in source code instead you have to store it other separate file and for this secret manager tool comes to be a handy tool. It helps to prevent sensitive data from being checked in by source control. ASP.NET Core also use this tool to manage user secrets during development. With secret manager tool you can associate app secrets with a specified project and share them across multiple projects. User secrets are stored as JSON data (like you see on appsettings.json).
As a developer you probably had to make user account system where you kept user login credentials (along with other personal details). If you are still using old style like storing password in plain text or using any other weak password encryption technique like AES then your user’s data is in risk. That’s an easy way for hacker. So hashing is another secure crypto technique/algorithm to implement for security.