For an intelligent IDE it is essential to be in the know about any external changes in files it working with - e.g. changes made by VCS, or build tools, or code generators etc. For that reason, IntelliJ platform spins background process to monitor such changes. The method it uses is platform-specific; and on Linux it is Inotify facility.
Before operate, inotify requires an "instance" to be initialized. While default per-user limit of instances used to be high enough, a growing number of programs which also use inotify may lead to instance shortage - in which case file watcher will be unable to start.
If this is the case, the limit may be temporarily increased by following command:
Don't forget to restart the IDE afterwards.