Stellar Authenticator is a tool for creating accounts and validating transactions on the Stellar blockchain. On this network, each one is responsible for its own accounts and Stellar Authenticator helps you with that. All your private data are kept encrypted on your computer and only your requests are sent on the internet.
You are a few clicks away from your Stellar account. All you need to do is to pick a username and a password. Please choose something that you will remember for sure. If you lose them, you won't be able to access your accounts anymore. There's no password recovery mechanism as nobody else will know about your username/password.
If you only want to see how Stellar Authenticator works, you can try the guest mode. Please remember that there's not much to do in there, as it is merely an authenticator for external Stellar services.
If you prefer to learn more about Stellar first, you can take a look at the official website, at stellar.org.
You can open a transaction that you'd like to sign using its URL or its XDR.