What does it mean to say that int enum patterns are compile-time constants?

This is from Effective Java

Programs that use the int enum pattern are brittle. Because int enums are compile-time constants, they are compiled into the clients that use them.

Can some one explain why the int enum pattern is called compiled type constant and what is meant by compiled into the clients?

Here s' an example of such a constant :

public static final int APPLE_FUJI = 0;