The idea is to grow female plants with big, sticky buds but no seeds (Sensimilla) - then the plant can put all her energy into producing those nice trichomes which is where the thc is. If a female plant gets pollinated by a male plant (or by going hermaphrodite), it will produce seeds, but little smokable bud. This is a big topic - I suggest you do a little reading in the FAQ section on the basics. GL!