Global is the keyword used in drools to define a global variable. The global variable is the one that will be visible to all the rules inside a DRL file.
Globals must be used very carefully in a DRL file since the changes in a global variable are not notified to the working memory. For example, you are using a list variable as a global and in any rule, you are adding value in a list, and in one particular rule, you are checking if list size() > 0, then in this case rule may not fire. A global variable can be used for any type of object.