Relaxing things are soothing: they make you feel calm, happy, and at ease.
Relaxing things are the opposite of things that create worry or excitement. For a lot of people, watching TV and reading are relaxing. Jumping off a building or taking part in a high-speed chase isn't relaxing at all — neither is taking a test. If something makes you feel mellow, then it's probably relaxing. It helps most people sleep if they do relaxing things first.