is there an actual way to get comfortable while meditating?

Not that you are necessarily dreaming of anything, but just the feeling as if you are in a dream. Everything is sort of slowed down, and you don't really have too many thoughts anymore. Yet everything also feels magnified.

I am always told that meditation is meant to teach you how to let go of thoughts, and most guided meditations tell you to let go of thoughts, but what does this actually mean, and how do you actually do it?

I have heard this said a few times but never asked why it is. Why is it important to have your knees below the level of your waist, is it for comfort, or another reason?