This keyword, like the breed, <breeds>-own, patches-own, and turtles-own keywords, can only be used at the beginning of a program, before any function definitions. It defines new global variables. Global variables are "global" because they are accessible by all agents and can be used anywhere in a model.
Most often, globals is used to define variables or constants that need to be used in many parts of the program.
Take me to the full NetLogo Dictionary