Allah (swt) here uses the Arabic style called (Qasr) which consists of negation (it is not) followed by exception (except) to indicate that the Holy Quran is not but a reminder. This style is used in other Surahs as well, such as:
وَمَا تَسْأَلُهُمْ عَلَيْهِ مِنْ أَجْرٍ ۚ إِنْ هُوَ إِلَّا ذِكْرٌ لِّلْعَالَمِينَ
(And you do not ask of them for it any payment. It is not except a reminder to the worlds.) (Yusuf: 104)
The word (reminder) indicates that the instinct of human beings should keep remembering of Allah (swt). As the heedlessness of events is a matter of fact, so Allah (swt) made the Holy Qur’an is the entire reminder.
( لِّلْعَالَمِينَ… ) (…the worlds): This is a plural noun. It refers to everything other than Allah (swt). There are world of the angels, world of the jinn, world of the humans, world of the plants, world of the animals. Some of these worlds have no Messenger because they do not have the freedom of choice.