To create a variable value use the Action Editor and select "Set a Variable".
Action Editor can be accessed on the Events tab of any Block.
Choose an intuitive name for your Variable and a value for that Variable. Here is a simple example
Once your variable is defined you give it a value:
Once a variable has been defined it is available in the Editor under the 'Variable' section
Once you click on the Variable name you'll see it in this format: [[var:MyVariable]].
This will allow that value to be accessed through your app and be used as whatever value is assigned to it at the current time.