Feminised seeds are cannabis seeds that, genetically speaking, produce exclusively female plants – although badly feminised seeds can result in hermaphrodite plants.
Genetically, a cannabis plant is more or less predisposed to become a male or a female. This is determined by two chromosomes, which everyone knows as the X and the Y chromosome. A plant with two X chromosome will be female, while a plant with an X and a Y chromosome will be male.
Feminised seeds are made by forcing a female plant to produce male plants by an application of gibberelic acid. This makes the female plant essentially a hermaphrodite and makes it possible for her to produce seeds from herself that only have XX female chromosomes.
Before these seeds can actually be sold as feminised it can take years of testing and cross breeding in order to be sure that they will produce plants that are definitely stable and consistent.
A hermaphrodite is a bi-gendered plant with both female and male flowers (seed sacks). A hermaphrodite is capable of fertilizing itself (and the plants around it!), resulting in seeds in place of smokable weed.