The Monitor tab has a graphical overview of the runtime, including CPU, heap, threads and loaded classes:
This tab is useful for "at a glance" information, leaving further drill-down for other tabs.
In the following screenshot, the Threads tab shows the status of all threads:
It is very useful to find out if any threads are undergoing contention, entering deadlock, are underutilized, or they are taking up more CPU. Especially in concurrent apps with in-memory state, and in apps that use limited I/O resources (such as connection pools, or network calls to other hosts) shared by threads, this feature provides a great insight if you set the thread names:
Notice the threads named citius-RollingStore-store-1 through citius-RollingStore-store - 4. In an ideal no-contention scenario, those threads would have a green Running status. See the legend at the bottom right of the image, which explains thread state: