Matter appears to our senses to be solid but underneath it all, it is energy vibrating and various frequencies to form patterns. These patterns reflect the underlying order of the Universe. This is what really happens when we bang our toe on a rock. Our physical energy patterns interact with the energy patterns of the rock. This produces a pain frequency of vibration, interpreted by the brain, which is operating at its own vibrational frequency of particles and waves.
Tesla is saying the the secrets—and they are secrets that can never be fully encompassed by our minds our our instruments—will always remain secrets. You can think in terms of energy, frequency, and vibration. But are you capable of fully grasping these terms? If you are, does that mean you will finally have the key to all of life and are now able to manifest all the goodies you want? Some people interpret his statement to mean just that. “If I vibrate at a certain frequency, I will get all the stuff I want and my life will be solved.”
Tesla had something deeper in mind that making our earthly existence easier. He was aiming at what Einstein once said, “I want to know the mind of God. The rest is details.” And, like Einstein, Tesla dealt in energy. They were delving deeper into the mysteries that will never be fully understood. And thus, they are secrets.