In experience design, friction is anything that prevents users from accomplishing their goals or getting things done. It’s the newsletter signup overlay covering the actual content, the difficult wording on a landing page, or the needless optional questions in a checkout flow. It’s the opposite of intuitive and effortless, the opposite of “Don’t make me think.”
Having said that, friction can still be a good thing sometimes. In game design, for example, friction is actually required.