Something that's restful soothes you or helps you to relax. At the end of a long, busy day, a hot bath can be restful.
Chasing your three year-old cousin around a playground in the hot sun for an hour isn't particularly restful, but you might follow that up with a restful nap in a comfortable hammock. Anything peaceful and quiet is restful. The word adds the suffix -ful, "full of" or "characterized by," to rest, which comes from a Germanic root meaning "rest, repose, or peace."