Build configuration on Travis CI is done via a .travis.yml file, which is committed to the repository.
This is generally handy, as the build configuration is versioned with your code. However, sometimes you want configuration options in there that should not be publicly visible. This is why you can encrypt these values. For security reasons, the encryption should happen on your machine, rather than transmitting the unencrypted value over the network.
You can accomplish this by installing our command line tool and using the encrypt command or right here on this website. This page will not send the plain text nor the encrypted value over the network.