When you create an Android application that will be uploaded to the Google Play store, you generate a unique identifier that will allow Google, as well as every phone that installs your app, to know that you developed it. It is like your own secret password that you shouldn't share with anyone. If a malicious developer creates an "update" to your app, and somehow were to distribute it as such to users, any user that tried to install it would receive an error message because the malicious fake upgrade would not have been created with your private keystore. When you release your app, you'll have to make this keystore, as well as use it to generate a Facebook key hash, which is what they are referring to here.
However, when you are developing and not ready to release to the world, by default Eclipse will build your application using the default "debug" keystore. The instructions that you are referring to tell you how to generate the Facebook hash with your debug keystore. Once you do that, whenever you build your app with the debug keystore and run it, Facebook will allow it to connect using the SDK, as long as you have pasted that key hash into the Facebook developer console. It does not matter whether you are running the app on your computer or phone, it will run and connect with Facebook integration.
When you are ready to release, you'll have to generate a brand new keystore, run the tool again on that file, and upload that key hash to Facebook as before. Then, your app can run on anyone's phone and be able to connect to Facebook, provided you build it with the new keystore.