Assuming you are talking about population/Logistic growth:
The environment only contains enough of a resource (food shelter, maters, etc…) to keep a certain number of individuals in a population alive. The s shape is caused by the population reaching number close to this maximum size limit (called carry capacity). As the population gets larger than can be supported, the population crashes (extra death) and then more births bring the total population back close to the carrying capacity.
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