Harvest is a mechanics that introduced in Harvest league.
Seed Harvesting
In order to start harvesting, you need at least 8 fully grown seeds (which take at least 3 grow cycles to grow). Once you have eight fully grown seeds press the harvest button on top of the lifeforce collector . The seeds you have grown will turn into dangerous monsters that you will have to kill immediately. Upon killing all of the monsters you will be rewarded with different one-time use crafting option (recipe) and loot, depending on the tier and monster level of the monsters and the type of monster planted. Its recommended to have a build with insane clear in order to make the monsters grown less of a hassle.