A global variable is a variable declared on the special Stage Actor, and is easily visible/accessible in ALL Actors in a project.
A local variable is a variable declared on any Actor that isn't the Stage Actor, and is only easily visible/accessible to that particular Actor.
If a local variable and a global variable share the same name, the local variable takes precedence (the global variable) is ignored.
There is also another type of variable called a script variable, which is only visible/accessible to code blocks in the same connected code block chain. They aren't even accessible to other scripts in the same Actor.