This is the 619th most frequent Estonian word.
maailm
The world.
In this sentence, 'maailmas' is the inessive form of 'maailm', indicating something existing within the scope or domain of the world.
Maailmas on palju erinevaid kultuure.
There are many different cultures in the world.
Here 'maailm' refers to the world in a general and literal sense.
Maailm on ilus ja avar.
The world is beautiful and vast.
'Selles maailmas' uses 'maailmas' (inessive form) with the demonstrative pronoun 'selles' to specify a particular kind of world, possibly metaphorical.