A plant seeds in order to reproduce itself. A seed must be pollinated to reproduce itself, and must be pollinated to produce a new plant. A self-pollinated seed fertilizes itself and is considered "easy" to save. Cross-pollinating seeds must share pollen from one plant to another to be fertilized. These seeds are labeled as "advanced" because it requires effort to keep them crossing with each other. "Medium" seeds fall somewhere in between because they can cross with other plants and their species.
In the Common Soil Seed Library catalog, the difficulty for saving seeds for each plant is categorized easy, medium or difficult. The levels are not indicators of how challenging it is to grow the plant, only the effort to save the seed. Examples of "easy" to save seeds include artichokes, beans and tomatoes. "Medium" seeds include beets, carrots and onions. Broccoli, corn and pumpkin are considered "advanced" seeds.
Once you've learned how to save seeds and have collected seeds from your healthiest crops, select some for the seed library and place them in clearly labeled envelopes or containers. Label the container using the seed donation form. It is important that seeds donated or returned to the library are saved properly to ensure that the integrity of each variety remains intact.