'The world most beautiful flowers' has no meaning. The apostrophe and s ('s) is there to indicate possession or belonging. They are the most beautiful flowers of the world (belonging to the world).

Maybe it would help to think of a smaller example than 'world'. Imagine a clothing shop. The best dress in that shop would be "the shop's most beautiful dress." "Shop's" is possessive because it's the best dress that the shop has.

It's an apostrophe of possession. A long time ago, it would have been written like this: The world: its most beautiful flower. Thus "world's" = "world: its".