An IllegalMonitorStateException is a runtime exception in Java that arises in multithreaded applications when a thread attempts to wait on or notify other threads waiting on an object's monitor without owning the monitor. This unchecked exception occurs when methods such as wait(), notify(), or notifyAll() of the Object class are invoked outside a synchronized block or method. An example is provided where calling the wait() method outside a synchronized block results in this exception, but moving the call within a synchronized block resolves the issue by ensuring that a lock on the object's monitor is acquired. Tools like Rollbar can aid in managing and resolving such exceptions by providing automated error monitoring and triaging, enhancing the confidence and efficiency of deploying production code.