Alam:
In Arabic language, ‘Alam translates to “world”. The plural of it is Awalim which translates to “worlds”. ‘Alam is derived from ‘Alamah, that is because it is a sign testifying to the existence of its creator and to his oneness.
‘Aalameen:
The closer translation to ‘Aalameen is nations. The “een” at the end in Arabic language is constantly a reference to creatures of intellect – human beings, angels, jinns etc. For example, muttaqeen, kafireen, saliheen etc.
Worlds include non-living things like rocks, mountains, seas etc. whereas nations include people. The people referred here in nations are people of different countries, different generations etc. Allah (subhanahu wa ta’ala) is the Rabb of all nations.
Allah (subhanahu wa ta’ala) mentioned at several places in the Quran that he is the Rabb of skies and the earth but in Fatihah he mentioned he is the Rabb of nations. Why?
It is because Fatihah is not about everything else. It is about us, so the idea is to think about how Allah (subhanahu wa ta’ala) owns us and not the sky or the earth. Islam and the Quran is a more personalized conversation. It starts with us and goes everywhere else. It is not philosophical.
Another reason Allah (subhanahu wa ta’ala) emphasizes only on living species here is because they are the ones with the intellect and all the living species can control or impact the non-living species.
Meaning of Rabbil ‘Aalameen
Rabbil ‘Aalameen implies that he is the master, the caretaker, the authority, the owner, the one who ensures development and growth, the one who holds together, every culture, every society, every generation throughout the world.
Allah (subhanahu wa ta’ala) wants every nation to develop and get better than it was before. That is implied in the word Rabb.