Automatic cannabis seeds do exactly as the name suggests, they produce cannabis plants that flower automatically!
The reason why the plant changes from its vegetative growth stage to the flowering stage, is because this change is based on its age, rather than the light cycle, like most photoperiod plants.

Automatic cannabis seeds are made up of two components, One is a regular cannabis strain, like an Indica or a Sativa, And the other is the lesser-known strain Ruderalis.
Ruderalis was discovered in 1924 in South Siberia by Russian botanist Janischewski. 

Unlike Indica’s and Sativa’s, Ruderalis has adapted its genetics to flower within 3 to 5 weeks due to the harsh conditions of its environment. It is able to cope with extreme heat, serve cold, and weak or excessive light.
Ruderalis was mainly ignored by cannabis smokers as it has a low THC content. But breeders in the mid-1990s looked into the plant as a way of cross-breeding existing feminized cannabis seeds and automating the flowering process.

Early automatic flowering plants had low levels of THC. But after many years of cross-breeding, today’s auto-flowering plants have high levels of THC and CBD, almost exactly the same as their photoperiod sisters.

Growing an automatic flowering plant has many advantages for the home grower as multiple crops can be grown in a single outdoor season. Plant species vary, but a plant can be grown from seed to harvest in as little as nine weeks, making this a very attractive way of growing.

Generally, automatic flowering plants are shorter than their non-auto flowering sisters, this is because of the Ruderalis genetics, which also means that there is no reason to change the lighting period on these plants.