You can find the definition of the terms corepoolsize and maxpoolsize in the javadoc. http://docs.oracle.com/javase/6/docs/api/java/util/concurrent/ThreadPoolExecutor.html
The link above has the answer to your question. However, just to make it clear. The application will creating threads until it reaches the corePoolSize. It means that these number of threads should be sufficient to handle the inflow of tasks. After that the tasks will be queued. Once the queue is full the executor will start creating new threads. It is kind of balancing. What it essentially means is that the inflow of tasks is more than the processing capacity. So, Executor will start creating new threads again till it reaches Max number of threads. Again, a new threads will be created if and only if the queue is full.
