With nothing much happening — no surprises, no trouble, no excitement. The important thing is the feeling behind it, which is usually relief, not boredom. An uneventful flight is a good flight. An uneventful night in a hospital means the patient is fine. An uneventful journey, an uneventful week, an uneventful recovery: in every one of these, the speaker is glad. That is because the things that could have "happened" would have been bad ones. So while the dictionary says "nothing exciting or unusual happening", the real message is closer to "nothing went wrong". It can carry a small sigh of boredom too — "my summer was pretty uneventful" — and you can hear which one is meant from the situation and the tone. It goes with stretches of time and with journeys most of all, and it is mildly formal: a friend would more likely say "nothing happened".