Some verses clear, others not.

هُوَ الَّذِي أَنْزَلَ عَلَيْكَ الْكِتَابَ مِنْهُ آيَاتٌ مُحْكَمَاتٌ هُنَّ أُمُّ الْكِتَابِ وَأُخَرُ مُتَشَابِهَاتٌ فَأَمَّا الَّذِينَ فِي قُلُوبِهِمْ زَيْغٌ فَيَتَّبِعُونَ مَا تَشَابَهَ مِنْهُ ابْتِغَاءَ الْفِتْنَةِ وَابْتِغَاءَ تَأْوِيلِهِ وَمَا يَعْلَمُ تَأْوِيلَهُ إِلاَّ اللَّهُ وَالرَّاسِخُونَ فِي الْعِلْمِ يَقُولُونَ آمَنَّا بِهِ كُلٌّ مِنْ عِنْدِ رَبِّنَا وَمَا يَذَّكَّرُ إِلاَّ أُوْلُوا الأَلْبَابِ

 It is He who has sent down to you (Muhammad) the Book (this Qur’an). In it are Verses that are entirely clear, they are the foundations of the Book [and those are the Verses of Al-Ahkam (commandments), Al-Fara’id (obligatory duties) and Al-Hudud (laws for the punishment of thieves, adululterers)]; and others not entirely clear. So, as for those in whose hearts there is a deviation (from the truth) they follow that which is not entirely clear thereof, seeking Al-Fitnah (polytheism and trials), and seeking for its hidden meanings, but none knows its hidden meanings except Allah. And those who are firmly grounded in knowledge say: “We believe in it; the whole of it (clear and unclear Verses) are from our Lord.” And none receive admonition except men of understanding.[Imran v.7] (tafsir tabari).

Allah states that in the Qur'an, there are Ayat that are Muhkamat, entirely clear and plain, and these are the foundations of the Book which are plain for everyone. And there are Ayat in the Qur'an that are Mutashabihat not entirely clear for many, or some people. So those who refer to the Muhkam Ayat to understand the Mutashabih Ayat, will have acquired the correct guidance, and vice versa. This is why Allah said (They are the foundations of the Book), meaning, they are the basis of the Qur'an, and should be referred to for clarification, when warranted, (And others not entirely clear) as they have several meanings, some that agree with the Muhkam and some that carry other literal indications, although these meaning might not be desired.

The Muhkamat are the Ayat that explain the abrogating rulings, the allowed, prohibited, laws, limits, obligations and rulings that should be believed in and implemented. As for the Mutashabihat Ayat, they include the abrogated Ayat, parables, oaths, and what should be believed in, but not implemented.

(So as for those in whose hearts there is a deviation) meaning, those who are misguided and deviate from truth to falsehood, (they follow that which is not entirely clear thereof) meaning, they refer to the Mutashabih, because they are able to alter its meanings to conform with their false interpretation since the wordings of the Mutashabihat encompass such a wide area of meanings. As for the Muhkam Ayat, they cannot be altered because they are clear and, thus, constitute unequivocal proof against the misguided people. This is why Allah said (they follow that which is not entirely clear thereof) meaning, they refer to the Mutashabih, because they are able to alter its meanings to conform with their false interpretation since the wordings of the Mutashabihat encompass such a wide area of meanings.

As for the Muhkam Ayat, they cannot be altered because they are clear and, thus, constitute unequivocal proof against the misguided people. This is why Allah said (He [`Isa] was not more than a servant. We granted Our favor to him.) [43:59], and (Verily, the likeness of `Isa before Allah is the likeness of Adam. He created him from dust, then (He) said to him: "Be!'' and he was.) [3:59]. There are other Ayat that clearly assert that `Isa is but one of Allah's creatures and that he is the servant and Messenger of Allah, among other Messengers.

Allah's statement (And seeking for its Ta'wil,) to alter them as they desire. Imam Ahmad recorded that `A'ishah said, "The Messenger of Allah recited (It is He Who has sent down to you the Book. In it are verses that are entirely clear, they are the foundations of the Book; and others not entirely clear,), until (Men of understanding) and he said,(When you see those who argue in it (using the Mutashabihat), then they are those whom Allah meant. Therefore, beware of them.)''

 Al-Bukhari recorded a similar Hadith in the Tafsir of this Ayah [3:7], as did Muslim in the book of Qadar (the Divine Will) in his Sahih, and Abu Dawud in the Sunnah section of his Sunan, from `A'ishah; "The Messenger of Allah recited this Ayah (It is He Who has sent down to you the Book. In it are verses that are entirely clear,) until (And none receive admonition except men of understanding.)

He then said (When you see those who follow what is not so clear of the Qur'an, then they are those whom Allah described, so beware of them.)'' This is the wording recorded by Al-Bukhari.

Taken from: tafseer Ibn Katheer