A static inner class is a class nested inside another class that has the static modifier. It's pretty much identical to a top-level class, except it has access to the private members of the class it's defined inside of.
class Outer {
private static int x;
static class Inner1 {
}
class Inner2 {
}
}
Class Inner1 is a static inner class. Class Inner2 is an inner class that's not static. The difference between the two is that instances of the non-static inner class are permanently attached to an instance of Outer -- you can't create an Inner2 without an Outer. You can create Inner1 object independently, though.
Code in Outer, Inner1 and Inner2 can all access x; no other code will be allowed to.
