The Roc's Reseed Mod is an addon that adds new functionality to the game. It allows you to acquire seeds from crops you have grown. The way this works is that when you harvest a crop, you now get a chance to acquire its seed. This is based on farming skill, with a default of about 5% with no skill and 33% with lv 10 farming.

You can modify the numbers governing these chances in the config file. The way this works is that you get a random number based on the farming skill of the player. If it is higher than 5%, you get a chance to acquire a seed from the crop.

You can see the amount of seeds you have in your inventory, and the amount you have harvested.