Global variables are those variables which remain same throughout the project. If ever the change in variable arises, then just making a change in the global variable would suffice. There is no need to change that variable in the whole project. These variables are user-defined that is, the user can make its own variables as per need and use them throughout the project.
For detailed information on Global Variables, please refer : https://opkey.crestechglobal.com/opkey-help/?subid=68&accordid=5#Global-Variables