Put most simply, they are strains of cannabis that will change from their vegetative cycle to their flowering cycle without any assistance or external input. What does that mean? Well, normal types of marijuana plant will start as a seedling and then grow in a “vegetative” period where they basically just get bigger. They grow deeper roots, a thicker stem and grow lots of big leaves to absorb sunlight. When growing in the wild, they will stay in that period over the course of the early summer until the days begin to get shorter. When that happens they start to grow buds (they enter their “flowering cycle”) in an effort to reproduce.
Autoflowering cannabis strains, do not need this change in light to make them flower, they will do it automatically after 3-4 weeks regardless of how much light they are getting.
For a full explanation of what an autoflowering marijuana plant is – click on this great blog article