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For every build that you make a new version for, you are able to put comments. These comments are useful to track the changes you have made to your application or to denote a specific version that you might like to use. It does not matter what you put in the comments and you can change them later.
Best Practice: Creating Application Checkpoints
While you are application building, it can be a good idea to create "checkpoint" builds throughout the process. Checkpoint builds are created to provide stable checkpoints in the application development process in case serious problems necessitate the need to revert an application completely.
For example, if you are actively building and testing a complex application, it may be a good idea to periodically deploy (but not release) a stable build. In the notes, you can write "this is a checkpoint build as of [date and time] by [your name]". Now, if you continue app building and run into errors that you are unable to fix or pinpoint why they are happening, you always have this checkpoint build you can revert to. You can revert an application by clicking on the "Revert to this Version" button on the application deployment page as shown below:
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This will change your application (including all forms and application settings) back to the way it was when you created this checkpoint build.
Publish an Application to a Phone
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Updating your application: Read more here: Update Your Application