A global variable cannot be reassigned. It will always point to the same object. However, its content might change, if the value is mutable (for example, the content of a list). Local variables don't have this restriction.
a = [1, 2]
a[1] = 3
b = 3
b = 4 # forbidden
ERROR: /path/ext.bzl:7:1: Variable b is read only
You will get a similar error if you try to redefine a function (function overloading is not supported), for example:
def foo(x): return x + 1
def foo(x, y): return x + y # forbidden