The phrase embodies a design philosophy - originally applied to software platform development but applicable to other places as well - that any infrastructure or platform should allow its users to "fall into the pit of success" without even trying, meaning that the default settings should be those that just work, rather than relying on an expert user to change and tweak the initial settings to make sure the system works properly.
The definition of the Pit of Success is taken from Jeff Atwood's blog:
The Pit of Success: in stark contrast to a summit, a peak, or a journey across a desert to find victory through many trials and surprises, we want our customers to simply fall into winning practices by using our platform and frameworks. To the extent that we make it easy to get into trouble we fail.
and attributed to Microsoft program manager and software performance guru Rico Mariani.
Atwood brings additional quotes from Microsoft language and API designer Brad Abrams about this:
[we should] build platforms that lead developers to write great, high performance code such that developers just fall into doing the "right thing". [...] We should build APIs that steer and point developers in the right direction.