The Hebrew word for "still" in Psalm 23 is menuha. Read the following quote by Rabbi Abraham Joshua Heschel:
To the biblical mind menuha is the same as happiness and stillness, as peace and harmony. . . It is the state in which there is no strife and no fighting, no fear and no distrust. The essence of the good life is menuha. . . . In later time menuha became a synonym for the life in the world to come, for eternal life.