3:7  He it is Who has revealed to you the Scripture, whereof some Āyāts are Decisive— these are the Mother of the Scripture, and others are allegorical. Then as to those in whose hearts there is perversity, they in fact follow the allegorical part of it seeking trouble and seeking to give it interpretations. But no one knows the interpretation of it except Allāh. And the steadfast in knowledge would say: “We believe in it,— the whole is from the presence of our Rabb.”  And no one minds except those possessed of understanding.

3:7 This verse provides the key guidelines for exegetical methodologies of the Qur’ān and more importantly rules of interpretation of the messages in a manner that would self-reflect upon its own teachings. The verse provides answer to such methodological issues in exegetical works, e.g. if the Qur’ān is uniform in genre or diverse, and whether the analogical explication can accommodate variations in the basic teachings. Some verses of the Qur’ān are called muhkam or decisive, which are termed as mother of the Scripture (ummul kitāb) and others are mutashābih i.e. allegorical or idiomatic. The decisive verses are fundamental and admit a definite meaning, consequently such verse can provide singular, unambiguous message. The word muhkam is derived from hakama (lit. to govern, to have power over, command, direct etc), thus, these verses relate to specific commandments, obligatory duties and legal pronouncements. The word umm, although translated literally as ‘mother’, implies what could be the essence, basis, principal or matrix (13:39: 43:4), thus, by itself sets an allegory emphasized in the other key word mutashābih. The allegorical or ornamental verses are those that are liable to provide more than one plausible expression; however, limiting them within the boundaries defined by the muhkam verses and negating scope of any wanton speculation (Rāzī). Thus, the meaning of the mutashābih verses cannot override the unambiguous message of the muhkam verses (39:23). Furthermore, the purpose of mutashābihat is not obscure the meaning of the Qur’ān, which by all means is clear and unambiguous, but to serve crucial didactic functions in the Qur’ān.   
Those who do not follow the above guidelines would create trouble or dissention (fitnah) by providing interpretation (ta’wil) out of their own desire in an arbitrary manner relying solely on the mutashābih verses without referencing the supportive principal, unambiguous verses.
The phrase no one knows interpretation of it except Allāh implies the interpretation of allegorical verses are beyond the perception of ordinary human intelligence. For example, the aspects of heaven and hell, resurrection, attributes of Allāh, eternal life, purposes of creation etc. are conveyed in immensely allegorical terms, often simplified in common parlance, yet outside the grasp of human cognizance. 
The Qur’ān praises those who believe in it i.e. the Qur’ān – the whole of it from the presence of our Rabb. Thus, it strongly recommends reading and interpreting its message holistically, therefore intratextually rather than reading it in selective and decontextualized manner. Only the steadfast in knowledge and those who possess understanding would believe in the whole of the message through proper study and research (cf. 38:29; 47:24) for Allāh made the Qur’ān easily understandable (26:195; 44:58). This phrase fortifies the fact that people possessing understanding can comprehend the muhkam and mutashābih verses. Al-Jawzi summarized the mutashābihat as divine test for the believers, a test that would spark their investigative interests. This aspect of mutashābihat indicates greater the efforts extended to understand the message the greater will be the reward (Rāzī, McAuliffe).