When Seeds Need Waking Up

Most domesticated vegetables and grains sprout when and where they are planted. They no longer go dormant (literally meaning "sleep") because the gardener is going to take care of them. Perennials, berries, trees, and many herbs are closer to the wild, and still go into dormancy to help them survive. There's a page of seeds needing stratification here.
Seeds go dormant in nature to wait until conditions are right for survival. To break their dormancy (wake them up), the gardener needs to create the conditions the seeds are waiting for.
There are three tricks gardeners use to break dormancy and get seeds to sprout: Stratification, Scarification, and Soaking.

Cold Stratification:
Many trees, shrubs, and many perennials from temperate climates like to go through the winter as seeds. Herbs like Valerian, Lavender, Nettle, and Echinacea behave this way. So do the wilder perennial vegetables like Caucasus Mtn Spinach, Lovage, and Good King Henry. In nature, they drop from the plant, spend the winter in cold wet soil, and sprout in spring.  This is an adaptation to temperate winters--it keeps the plant from having to face freezing temperatures as a small seedling.
The process is called cold-conditioning or stratification. You need to give the seeds enough cold time to meet their chill requirement--which is given on the packet and the website for each variety. If you plant in pots in the fall and leave them outdoors, they will sprout when the weather warms. If you want to plant in summer, you can use the refrigerator to provide the cold. (Place the seeds in a bit of moist potting soil in a jar before refrigerating, then plant as normal at the end of the stratification period.) You can see many seeds that need stratification here.
Each plant has a specific length of time that it needs for this cold, moist period--this prevents them from sprouting during unseasonable thaws. So, for example, Rowan, or Mountain Ash, which naturally grows around the world in northern regions, needs 90 to 120 days of cold before sprouting. Red Root, from Mediterranean climates in the western US, needs only 30 to 90 days of cold. And Moringa, from the tropics, needs none. It doesn't even like to be stored in a refrigerator, because it needs warmth to stay alive.

Scarification:
There is another process that you may see mentioned on seed packets and in books. It is called scarification, and it is another way of breaking dormancy.
Cold is not the only danger to young seedlings--in many places, water is the limiting factor. A plant that sprouts before there is a good supply of water is condemned to shrivel and die. So, plants from places with a pronounced dry season have very hard seed coats that prevent water from entering the seed and initiating germination. These seeds need to have their tough seed coat scratched so that water can enter before they can sprout. In nature, flash floods or raging rivers tumble the seeds along with rocks and sand, which scratch the coating and start germination. That way, a tiny bit of rain or dew won't fool the seed into sprouting before there is enough water for growth. Licorice and Astragalus are both river bank-adapted herbs that need to be scarified.
You can scarify seeds easily at home with a piece of fine sandpaper. Just cut the sandpaper in half, put a few seeds on the paper, and put the other half on top. Rub them lightly together 3 or 4 times. Then try soaking the seeds. If they swell up within a couple of hours, they are adequately scarified. If not, try again. If the seeds are cracked or broken, use less pressure or fewer scratches next time. Usually one or two tries will find the happy medium.

Soaking:
Plants that don't encounter turbulent storm waters have another way of preventing premature germination. Instead of a tough seedcoat they have a coating of natural chemicals that prevent germination. Many dryland wildflowers, such as lupines, have this strategy. Garden plants that  have germination inhibitors are carrots, parsley, fennel,  beets, and especially parsnips. These seeds all need a good soaking to remove the germination inhibiting compounds. For example, carrots and parsnips often take two weeks to sprout in the garden, and need to remain moist the entire time. However, if they are soaked for 8 hours and rinsed well in a strainer, you can plant them and get little seedlings in a few days. The soaking removes the inhibitors that otherwise would dissolve very gradually in the moist soil. Be aware: most vegetable seeds do not have these germination-inhibiting chemicals and should not be soaked for long periods. (They will rot.) Peas, beans, and spinach can be soaked, but only long enough to soften the seed coat--usually 1 or 2 hours is plenty. Most other vegetable seeds need no soaking at all.