Eww is a sound people make when something feels dirty, slimy, or disgusting — a hair in the soup, a bad smell, a film scene they cannot watch. It is very informal: children and young people use it a lot in speech, messages, and comics. You would not put it in a school essay or a work email. Though actually, the number of w's is just how strong you want it to sound (ew, eww, ewww); it is the same noise. It sits with yuck and ugh.