As far as I can tell, the Mooer Black Secret is a clone of the RAT distortion pedal made by ProCo.
In addition to the standard RAT functionality, the Black Secret has a switch to change the distortion type. The label on the pedal indicates "Vintage" and "Turbo". Presumably, the switch allow the selection between standard RAT and Turbo RAT.
Overall, the pedal performs well. To my ears, it is indistinguishable from the RAT.
It also has a very small footprint compared to the ProCo RAT but it doesn't allow internal battery power. Moreover, compared to the RAT, the Black Secret uses a standard/Boss power socket. The RAT uses a 3.5mm jack.
If you look for a distortion pedal that uses a RAT (or RAT-like) distortion circuit, the Black Secret is a good choice. It's cheaper than the RAT, it uses a standard power socket and it has a switch to select standard/turbo clipping. If you can live without the internal battery supply, it's a no-brainer.