It's a bit confusing. There are two common meanings to the word "monitor". First, "monitor" is an antique word that means the same as modern "mutex" which is also the same as "lock" (noun). Note that the word "lock" can be used both as a verb and as a noun: you can lock a mutex and unlock a mutex, and lock a lock, and unlock a lock.
Second, in Apache code "monitor" means a combination of a mutex and a condition variable (which is an object that you can use for wait()/notify()). So, technically, the Java objects are monitors in this sense, since you can do both synchronized() (e.g. lock/unlock) and wait()/notify() on them.
And, I guess, third meaning of "monitor", used in this book, is the same as is commonly called "critical section".
Because of this confusion, the word "monitor" is best not to be used.
So, the airplane lavatory is a "critical section": place that normally can be occupied by only person at a time. The lock on the lavatory's door is a "mutex": a device that let's the current occupant to be undisturbed after it's locked and until it's unlocked. If other passengers want to use the lavatory, they find the door locked and queue up waiting for it to get unlocked, taking their turns.