The following article explains what global variables are, what you can do with them and how you can create them.
What are global variables and HowTo create one?
1.1 What are global variables?
Global variables can be used throughout the entire application. These are useful when a specific value is used for multiple actions and is subject to change.
Usecases: a key for a webservice, an emailaddress from which you send mail in your mail actions, a signature you place under your emails.
1.3 How to create one?
You can create new global variables by going to variables in the tools menu. You have to give the variable an easily understandable name, select a type and set the value.
Global variable values are not variable between sandboxes. If you need to use different values for your environments you'll have to create a custom configuration. This configuration then can be used as variable in the global variable. Custom configurations have to be given a value in each environment separately. You can find more about this here.