Cannabis is grown from one of two sources: a seed or a clone. Seeds carry genetic information from two parent plants that can be expressed in numerous different combinations, some like the mother, some like the father, and many presenting various traits from both. Generally, commercial cannabis producers will plant many seeds of one strain and choose the best plant. They will then take clones from that individual plant to get consistent genetics for mass production.
But for the typical homegrower, it may be easier to obtain seeds rather than clones. Growing from seed can produce a stronger plant with more solid genetics.