Seeds, seeds and more seeds. Millions, billions of them being generated all over the world; anywhere that seed bearing plants can grow (which is virtually everywhere).
 First the seed has to be dispersed. Some simply drop or are light enough to borne by the wind, some are dispersed by water. Some are digested by animals and excreted elsewhere. Some are moved in other ways by animals. Some seed pods literally explode and propel their seeds a distance. I suppose we should include humans buying seeds and planting them.
 At the time of dispersal the seed is dormant and breaking dormancy is a timer of sorts for a seed to germinate at the right time e.g. once winter has passed. This dormancy may be physiological (responding to certain chemical factors or light or temperature), morphological (the embryo has to mature further), physical (the seed has a hard coat that needs to break down) or mechanical (certain chemicals need to be washed out of the seed by rain or snow).
 Nature will work its course but sometimes gardeners mimic the process e.g. by soaking seeds or by freezing them (stratification) or by physically damaging the coating with abrasion (scarification). Whether by natural or artificial means the seed can then germinate in response to water, temperature, oxygen and the presence or absence of light (some seeds need to be underground and some on the surface).
 Of course not all seeds will germinate and many seedlings will not survive prevailing conditions much beyond germination. The majority of plants account for this by massively overproducing seeds like the Aster going to seed in the photo above. Many seeds can remain viable for extended periods awaiting the right conditions, years and decades is not uncommon. Researchers have successfully germinated ancient seeds; one of the oldest is a date palm in Israel grown from 2,000 year old seed.