The noun relaxation describes the act of making something less strict. If your boss announces a relaxation of the company dress code, that means lots of happy people will be wearing jeans to work.
Relaxation carries many shades of meaning. It can describe being free from responsibility, like a relaxation of your usual obligations that comes while you're on vacation. The feeling of rest and peacefulness that can come from this freedom is also called relaxation. It comes from Latin: re- means "again," laxare means "loosen," and the suffix -ation means "the state of." So relaxation is the state of loosening something again.