The All is part of the Hermetic philosophy as outlined in the Kybalion but similar to concepts seen in many other philosophies and traditions throughout history. The All is everything. Everything that is, was and could be. Real things, unreal things, truths, untruths. We all exist within the All, and the All exists within us.
For some, the All is God. For some, the All is Chi. For some the all is the element of Spirit or Aether. For some the all is the energy of the Universe that makes up all things. For some, the All is all of these things. Or something else. Above all, the All is Unknowable.