An unusual effect is a effect on all unusual quality hats. Basically anything weird or questionable floating around your head, for example: fire, snow, electricity, plasma, metal objects like screws or bolts etc. This tag normally goes with team fortress 2 pictures. If you want to see all the unusual effects in tf2 then copy and paste this link for more info.

https://wiki.teamfortress.com/wiki/Unusual
unusual effect