So a randomizer is when you take an existing video game and reshuffle the items and progression in it, so it plays differently every time
A mystery randomizer is when the player isn't told what the settings are (for example, what can be reshuffled, what stays the same, and what the goal is) and has to find them out by playing
And a mystery mystery randomizer is when I don't know what mystery pool the settings come from, or whether it's some other one-off randomizer mode, or even necessarily what game it is!
I have a Mystery Mystery pool of randomized seeds that's usually A Link to the Past (z3r), possibly Earthbound (PK Scramble randomizer), and occasionally Illusion of Gaia randomizer or Super Mario World randomizer. They're all Super Nintendo games so they all look the same when you rename the ROM files after their hash.
This is a thing I stream frequently around now -- usually starting sometime around 9 pm Eastern on Sundays, Tuesdays, Wednesdays, or Fridays, though this isn't any kind of firm schedule