|All these tiny seeds from one poppy!|
Cold stratification subjects the seeds to cold and moist conditions for one to three months (varies with species), either outdoors in the natural environment, or in similar conditions you create indoors. Without this exposure, the seeds will not germinate, so this requirement dictates the timing of next season's growth.
Take wildflowers as an example. If my wild poppies flower in May, I will leave the spent flower on the plant to create seedpods which ripen in late June. The pods bend over and dump hundreds of tiny seeds on the ground. If these seeds were to germinate right away, new young poppy plants would start growing right away and flower again in a month or two. But Mother Nature didn't intend these wild poppies to be flowering again in late summer. To prevent the dropped seeds from germinating immediately, cold stratification is required before wild poppy seeds will germinate. So they must be exposed to winter weather, sitting dormant in the soil before they can germinate and begin to grow the following year. So if I gather those seeds in June, store them in my 60°F basement in an envelope until next spring, they most likely will not germinate when I plant them.
For this reason, I commonly sow my wildflower seeds between September and December, directly on the soil where I want them to grow. I'm not fussy about this, since they'd normally just drop off the plant, so the most I might do is to rake the soil to roughen it, then throw the seeds out of my hand. This is how I scatter my collected wildflower seeds - poppies, black-eyed susan, oxtail daisies, blanket flower, larkspur, columbine, Mexican hat and others. This gives me a great crop of little seedlings in late winter, and they are strong little plants by the time the spring temperatures warm up.
But if you want to harvest and store seeds which require cold stratification - many seed catalogs and other online sources can identify the plants which require this for germination - and plant them early in the next season, you can artificially simulate nature's cold stratification process. Here's how:
1) Clean the seeds so no other plant materials (leaves, pods, etc.) are mixed with them
2) Place cleaned seeds in a zip bag with a paper towel or sterile vermiculite or sand, slightly dampened moistened but not so wet that mold will form. A little powdered fungicide can be added.
3) Store the zip bag in the refrigerator, preferably in the veggie/fruit compartment (temperatures from 34 to 41°F are recommended). Do not put in the freezer. Length of time in the fridge varies; do you research online or mimic your winter conditions.
4) Check seeds regularly for fungus or mold or early germination
Read more about cold stratification and about saving heirloom seeds.