At the most basic level, the Second Secret could easily be considered nothing less then the key to life itself. That's an exaggeration in many ways, but the creations that humanity were able to build with it after its discovery were nothing short of incredible. Although the Sixth Secret can in many ways mimic the Second Secret's abilities, at first glance it appears hampered by the complexities of needing to program the nanotechnology to execute its particular role. In truth, this limit should probably also apply to the Second Secret, but it doesn't.
The systems created by humanity before the Week of Sorrows are said to have been immensely powerful, and yet almost childishly simple. You could tailor organisms to specific roles, or gear them more towards multiple, and the system itself allowed for behavioural tweaking both in the design stage and even afterwards - although the following was swiftly deemed utterly illegal for any creation that could be considered sentient.
The Second Secret also was the basis for the widespread genetic biomods that the majority of humanity currently benefit from. Although Prologue has its own benefits, it suffers from an inability to affect change on Potentials. Second Secret biomods, on the other hand, suffer no such limits; they take effect long before a human Awakens as a Potential, and as such those modifications are seemingly treated as part of what the Potential's physical vessel 'should' be.
This Secret is responsible for a vast swathe of changes in human society that were only accelerating before the Week of Sorrows brought it to a screeching halt. It was it that allowed humanity to create the Dragons, in so doing sowing the seeds of the Potentials, and also remains responsible for many of the changes that the Elder First and other groups brought forward in the years since the Week of Sorrows. Given how it’s used by the Shiplords, however, this Secret currently rests in a place beyond taboo within the human psyche.