What is spectral flow symmetry?

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I can't find much about this, and am looking into this to satisfy personal curiosity. I will like to know what spectral flow is, and what spectral flow symmetry is. I tried looking for this on Wikipedia but there is nothing on this. There are some papers that seem to use this, but I am looking for a stupid simple description of this stuff that still retains full meaning of what it really is. If there is some simple example of this, it will be really good. I am curious as to how this is used in physics.