People aren't giving you a plain, simple answer.
Here, fair is being used as a noun. Merriam-Websters online dictionary give the following definition of "fair" used as a noun, in the meaning it has in your sentence:
Quote:1: a gathering of buyers and sellers at a particular place and time for trade
2 a: a competitive exhibition usually with accompanying entertainment and amusements b: an exhibition designed to acquaint prospective buyers or the general public with a product c: an exposition that promotes the availability of services or opportunities <health> <job>
3: a sale of assorted articles usually for a charitable purpose (this definition was edited by me for clarity)
"World fair" (also "World's Fair," or "world's fair) has the special meaning of a fair at which people from all over the world to show the products which they hope to sell, as well as to enjoy the entertainment. In the United States, famous world fairs include the Columbian Exposition in Chicago in 1893, the St. Louis World's Fair in 1904 (technically speaking, it was the Louisiana Purchase Exposition) and the New York World's Fair in 1939, which was a spectacular failure. There have been literally dozens and dozens of "world's fairs" since the 18th century (Wikipedia lists an exposition in London in 1756 as the first "world's fair").
So, your sentence is misleading in that it writes "world fair," rather than "world's fair." But to a native English speaker, the meaning of world fair is immediately evident. It means an exposition of products and a site for entertainments which claims to have gathered exhibitors from all over the world.
I can't think why the other people here have made this so needlessly hard for you. If you don't yet understand what a fair is, as a noun, in this context, i'll try to give you a better explanation.
Noddy, you should be ashamed of yourself for coming up with that airy-fairy crap when this person is trying to learn English.