The most likely situation is that your project will connect to an external service using some credentials, for example, a database. Where will you store all this information? The most common way is to have a configuration file inside your source where you place all your credentials. The main problem with this approach is that you will commit the credentials, and any person with access to the source will have access to them. It doesn't matter that you trust the people who have access to the repo; it is not a good idea to store credentials.
If you can't store credentials in your source code, you are probably wondering how you will store them. You have two main options:
Environment variables
External services
Let's take a look at each one so that you can choose which option is better for your project.