Avoid unitialized variables because they can have any value. Instead initialize them before using them. If you're assigning them directly without using it, that's fine. Some compilers will generate a warning if you don't initialize variables and some compilers will also throw an error if you use an uninitialized variable when running your app.
But what is this undeclared? Variables are declared before you use them:
Variable prime is now declared. You can't use variables without declaring them first:
Or was there something else you wanted to know?
x = 7; // OK, variable declared.
y = 6; // Not OK, variable not declared; compiler will complain.