Seeds are items that will increase certain stats. Every player starts off with three seed slots and, through the use of Seed Booster items, can add an additional six seed slots for a total of nine slots. Currently seeds come in small, medium, and large varieties.
Most small and medium seeds can be obtained by defeating Minibosses or as rewards from certain quests. Large seeds and a few medium seeds can be obtained through vendors located in the Black Sanctum in exchange for Sanctum Badges or vendors located in Ardan's Casino in exchange for Casino Tokens.
As of patch 1.52 individual seed slots can be 'cleaned', which removes a seed from that slot and frees it for another seed. To clean a seed slot, click the button then click the 'X' located to the left of the seed you would like to remove.