It is important to understand the different between right and wrong effort. Skillful effort applied to meditation leads to more wholesome states of mind, deeper peace, tranquility, and joy, and the arising of insight into the nature of the mind. Wrong effort leads to further stress and suffering. Meditation is a practice that requires the development of skill, because you want to do it right. Even "letting go" requires effort. In fact, it is much more difficult for many people than it may sound, as it is something that most people have never done and goes against the dominant habitual tendencies of the mind. Once you get there you can let go of the effort, but until the practice becomes effortless, it is required, no different than the development of any other skill. Effort doesn't mean that stress and strain needs to be involved, it simply means that one applies their mind in a skillful way. In fact, skillful effort, by definition, necessitates the absent of stress and strain. Otherwise it is not skillful.