Seasoning are herbs/leaves/seeds. You can find some in summer/autumn near...well everywhere. Plants can be threshed into seeds and leaves via the agriculture skill. Seasoning is optional for several receipes when cooking. Also herbal beverages produce by cooking : "boil / herbal beverage" can be used as seasoning, this way you can make a lot more meals out of only a few leaves.
examples of items that can be used as seasoning