Back when I was a Mechanical Engineering student I often studied friction. It was usually the enemy. It made parts wear out, equipment less efficient, and formulas less accurate.
But occasionally I'd see examples where its properties were used as a tool. The damper preventing doors from slamming. The brake pads on a minivan. The slipstream that causes a knuckle-ball to drop.
Friction may be an artifact of the physical world, but the analogy translates to the digital. Friction is the CAPTCHA a person must complete before submitting a comment. It's the time it takes for a new webpage to load. It's a submit button, when submission could have been handled automatically via ajax. It's scrolling the page to complete a form.
As an interface designer, you can manage friction as a tool, eliminating it on the workflows you want to encourage, and adding it on the workflows you want to discourage. Amazon.com allows you to make a purchase with just one click, but it takes 4 clicks and a form submission to get a phone number to contact customer support. You may agree or disagree with that choice, but you can't deny that it keeps sales moving and support requests down.
Friction going to be there, no matter what you do. How might you use it to your advantage?