Generally, the word sutra means aphorisms and manuals, mostly philosophical but not limited to, written by the great sages of Hinduism after the Upanishads.
Among all remember the Brahmasutra, that the Book of Brahman, the basic text of the Vedanta, Yogasutra by Pantnjali of the Book on the philosophy of Yoga, but also the Kamasutra of Vatsyayana, the Book on love and loving behavior (kama in Sanskrit means love and Kama is the god of love). Generally they consist of aphorisms, short phrases, lines, formulas sutra just, for the storage of pupils.
From an etymological point of view, sutra means string and comes from the root syu / siv, from which the Latin suere (sewing), suture (stitching), english to sew.
Most likely, the great treatises of Hinduism are called sutras, or strings, because at the time they were written on palm leaves and papyrus, which were then held together with just a thread.
And sutra is one of the names given to the string that the Brahmans lead to shoulder for life after the celebration of the sacrament or Upanayana samskara, which marks the second birth for those born in the upper castes.