You can set environment variables for a project by adding a special type of Integration. Go to Integrations in the left sidebar, and then either modify an existing "Environment variables" integration or add a new one:
Don't forget to connect to the newly created "Environment variable" integration.
Environment variables are a secure way to store secrets. They are stored encrypted, and decrypted only before mounting them into your project.
You might be storing your data in a database that does not yet have a native integration in Deepnote. Or perhaps you need to use a third-party library that issues an API key (e.g. Mailgun).
In this case, you can add an environment variable which will let you access the remote integration or third party service. An example is Deepnote's neptune.ai integration.