To rest means to relax into something and let it support you. Rest yourself on the couch for a while, if you're tired.
The word rest comes from the German rasta meaning "league of miles." If you walked that far, you'd need a good rest too. Whatever way you choose to use the word rest — whether it's something you do, something you take, or something you put your arm up on — you're describing a moment of stillness and calm. Rest also means "the stuff left over." Who will eat the rest of the cookies when you have to rest from eating so many of them?