Cold stratification is the process of subjecting milkweed seeds to both cold and moist conditions as they require these conditions before germination will ensue.

In the wild
In the wild, seed dormancy is usually overcome by having its hard seed coat softened up by frost and weathering action during winter. This is nature’s form of “cold stratification” or pretreatment. This cold moist period triggers the seed’s embryo; its growth and expansion can then break through the softened seed coat in its search for sun and nutrients.

In its most basic form, when the stratification process is controlled, the pretreatment amounts to nothing more than subjecting the seeds to storage in a cool and moist environment. This period of time may vary from one to three months.

To accomplish this seeds are placed in a sealed plastic bag with moistened vermiculite (or sand or even a moistened paper towel), which is then refrigerated. Three times as much vermiculite as seeds is used. It is important to only slightly dampen the vermiculite, as excessive moisture can cause the seeds to mold in the bag.

Soaking the seeds in cold water for 6–12 hours immediately before placing them in cold stratification can cut down on the amount of time needed for stratification, as the seed needs to absorb some moisture to enable the chemical changes that take place.

Following proper stratification, the seeds are ready to be removed and sown in the nursery bed for germination.

Alternatively, the seed may be sown in small pots filled with moist soil and then the whole thing enclosed inside a plastic bag before placing inside a common refrigerator.

Source: Wikipedia