Germination is the name of the process by which a plant starts growing from a seed.
Generally, plants produce seeds to ensure a next generation of the plant. Plants have various strategies to ensure that the seeds are in the right place in order to germinate (also called "sprouting") and grow.
Many seeds have simple needs, and just need the right temperature and moisture conditions to grow.
Others are more picky in what they need to break out of the dormancy that they are in when they are ripe. The seeds of many perennials have peculiar needs in order to break out of their dormancy period and start growing.
Some seeds require part of the shell to be physically abraded in order for the seed to be able to detect the right growing conditions. An example of this is a desert plant called Ocotillo: rainfall in the deserts this plant is native to will result in flash floods. The seeds get vigorously rubbed against rocks and sand in the flash floods, like sandpaper, and only then will the seeds germinate.
Some seeds need to pass through an animal's digestive tract, so they will need a weak acid to scarify the seed coat. Examples of this include Date Palms. Planting the seeds directly will not germinate, however when they pass through a desert animal's digestive tract, they will germinate. One can replicate the chemical process by sucking on the seed for 45-60 minutes (there is probably a better mechanism, but this is how I've got mine to germinate).
Some, like ginseng, need repeated warm/cool/moist cycles in order to decide that it is the right time to grow. The natural habitat for ginseng plants is in deciduous forests. As a result, they need to be stratified in a way that simulates the seeds being buried under leaves for 2 years.
