Someone who's laid-back is relaxed and at ease. A laid-back dog owner always stays calm, even when her dog jumps up on the dining room table.
The opposite of laid-back is uptight. While a laid-back babysitter pretty much lets his charges do whatever they feel like, an uptight babysitter might make a schedule for doing chores, eating, and a strict bedtime. A laid-back party is often much more fun than a formal, dressy event, since people can wear whatever they're comfortable in and enjoy chatting with each other. Laid-back was coined in the 1970s, possibly from the slouchy posture of motorcycle riders.