Quran is a book that is the word of Allah (God) and there is no doubt about its divinity. The claim is made by no one but Allah Himself.

This is the Book (the Quran), whereof there is no doubt, a guidance to those who are Al-Muttaqoon (the pious and righteous persons who fear Allah much (abstain from all kinds of sins and evil deeds which He has forbidden) and love Allah much (perform all kinds of good deeds which He has ordained)). Quran (Surah Al-Baqarah, Verse 2)

Allah makes it clear in the Quran that it could not have been revealed by anyone other than Him. He says:

And this Qur’an is not such as could ever be produced by other than Allah (Lord of the heavens and the earth), but it is a confirmation of (the revelation) which was before it [i.e. the Taurat (Torah), and the Injeel (Gospel)], and a full explanation of the book (i.e. laws, decreed for mankind) – wherein there is no doubt – from the Lord of the ‘Alamin (mankind, jinn, and all that exists). Quran (Surah Younus, Verse 37)

why Quran is it important?

Because Quran is a book that discusses mankind (and their guidance)

Indeed, We have sent down for you (O mankind) a book (the Quran) in which there is Dhikrukum,(your Reminder or an honor for you i.e. honor for the one who follows the teaching of the Quran and acts on its teachings). Will you not then understand? Quran (Surah Al-Anbiya, Verse 10)

Nothing like Quran can be written or produced
In the Quran, Allah has challenged mankind and the jinn to produce something like it, even one surah or one aayah (verse), but they could not do that and will never be able to do that, as Allah says:

Allah challenges people about producing like Quran

Say: “If the mankind and the jinns were together to produce the like of this Quran, they could not produce the like thereof, even if they helped one another.” Quran (Surah Al-Isra, Verse 88)