When you are using a third-party api in your application code. it usually requires you to supply some secret client information when making requests. if your code is opensource or accessible by a lot of people, putting these keys directly in code in plaintext can be a security issue.
One approach that i have seen in the wild is to use os environment variables to store the secrets and your app code can query the environment when starting up. this still requires someone to set up the environment in your server when