The BrahmaSutras are part of the prasthatraya, the three sacred texts. It is said that a person wishing to start a new school of thought must write commentaries on the prasthanatraya as every Vedantic school builds from these. The prasthanatraya consist of the Upanishads (srutis, or revelation), the Bhagavad Gita (being the most important text of the smriti, recollection) and the BrahmaSutras (being of the category sutra, system).
The word sutra itself is etymologically related to sew and thread and hence means some kind of memory aid between different texts or thoughts. The sutra from Vedantic schools does not mean sutra the way Buddhist schools mean it. The BrahmaSutras are said to be terse and initially seem to contain contradictory thoughts. Swami Krishnananda says the following about them:
Sutras are concise aphorisms. They give the essence of the arguments on a topic. Maximum of thought is compressed or condensed into these Sutras in as few words as possible. It is easy to remember them. Great intellectual people only, with realisation, can compose Sutras. They are clues or aids to memory. They cannot be understood without a lucid commentary (Bhashya). The commentary also is in need of further elaborate explanation. Thus the interpretations of the Sutras gave rise to various kinds of literary writings such as Vrittis (gloss) and Karikas. The different Acharyas (founders of different schools of thought) have given their own interpretations of the Sutras to establish their own doctrines. The Bhashya of Sri Sankara on Brahma Sutras is known as Sariraka Bhashya. His school of thought is Kevala Advaita. The Bhashya of Sri Ramanuja who founded the Visishtadvaita School is called Sri Bhashya. The commentary of Sri Nimbarkacharya is known as Vedanta- parijata-saurabha. Sri Vallabhacharya expounded his system of philosophy of Suddhadvaita (pure monism) and his commentary on the Brahma Sutras is known as Anu Bhashya.
Also the following about the need to write commentaries on the classical texts:
Those who wish to study the philosophy of Vedanta should study the Ten Classical Upanishads and the Brahma Sutras. All Acharyas have commented on Brahma Sutras. This is a great authority for every philosophical school in India. If any Acharya wishes to establish his own cult or sect or school of thought he will have to write a commentary of his own on Brahma Sutras. Then only it will be recognised.
Writing the commentaries on the PrasthanaTraya started from Sri Adi Sankara, who started the Kevala Advaitic school. Sri Swami Krishnananda uses the term uncompromising monism, though I think Advaita or non-dualism is possibly a better word. The following is also from the introduction.
According to Sri Sankara, there is one Absolute Brahman who is Sat-chit-ananda, who is of an absolutely homogeneous nature. The appearance of this world is due to Maya - the illusory power of Brahman which is neither Sat nor Asat. This world is unreal. This world is a Vivarta or apparent modification through Maya. Brahman appears as this universe through Maya. Brahman is the only reality. The individual soul has limited himself through Avidya and identification with the body and other vehicles. Through his selfish actions he enjoys the fruits of his actions. He becomes the actor and enjoyer. He regards himself as atomic and as an agent on account of Avidya or the limiting Antahkarana. The individual soul becomes identical with Brahman when his Avidya is destroyed. In reality Jiva is all-pervading and identical with Brahman. Isvara or Saguna Brahman is a product of Maya. Worship of Isvara leads to Krama Mukti. The pious devotees (the knowers of Saguna Brahman) go to Brahmaloka and attain final release through highest knowledge. They do not return to this world. They attain the Nirguna Brahman at the end of the cycle. Knowledge of Nirguna Brahman is the only means of liberation. The knowers of Nirguna Brahman attain immediate final release or Sadyomukti. They need not go by the path of gods or the path of Devayana. They merge themselves in Para Brahman. They do not go to any Loka or world. Sri Sankara’s Brahman is Nirvisesha Brahman (Impersonal Absolute) without attributes.