What Is A Seed Bank?

It's a repository for different types of plant seeds.  These are kept at very low (but above freezing) temperatures to prolong their viable life.  Ideally, custodians of seed banks re-germinate the plants (sow them again from seed) at set intervals to try to keep the seeds in store as fresh as possible. The interval depends on species; some plant seeds stay viable while dormant for much longer than others.

Although we hear a lot about animal extinctions, it's not as well known that many plant species are also rapidly going extinct.  Some of these are plants that have never been described or "discovered" by the modern world, at all. Some may have had beneficial pharmacological chemicals in them, but we may never know.

Seed banks are also essential to preserve varieties of food crops.  Human populations today are becoming heavily dependent on relatively few food stuffs.  It might only take a small number of plant pathogens to wipe our our main food crops and lead to food shortages.  Preserving different varieties of agricultural crops, with different levels of resistance to different types of pests, may be essential to safeguard our food supply.