Yeah, it's simple enough once you understand the structure. The included walkthrough does a pretty good job at explaining the way the events editor works, so you might want to take a look at it if you haven't already (It's part of the FRED help files accessed from the Help menu).
A boolean value is either true or false, nothing else. In the context of FRED, a boolean operator is something you use to check the status of something. For example, is-time elapsed is a boolean operator. It's true once the specified time has elapsed, false until then, so you can use it to check how far the mission has progressed and thus time an event.