A jelly is someone who checks out the HELP PAGE to find out what the site name means before asking what the hell the site name means. ;-)
Your answer is at the bottom of the page:
Where did we get the name? A fluther is a group of jellyfish, like a gaggle of geese or a run of salmon. “Fluther” is pronounced /‘flʌ ðɚ/, and rhymes with “brother” and “mother”.