He’s talking about the purple circle. Im sure you’ve noticed that the fiber discs have tabs that slide into those slots. Then when your clutch basket spins, those tabs will bang back and forth and over time wear grooves.
Yep, that's it. I was just starting to mark the same on your picture. There are 12 finger's, and a total of 24 sides.Look in the red circles I drew on your pic. You can clearly see at least 6 marks on each arm where the clutch plate has worn.
That surface needs to be smooth. Two things happen here - as your clutch is spinning the plates are riding against the arm where those grooves are and the clutch fibers are locked together in place by springs compressing them into the basket.
When you pull the clutch lever to disengage, the spring tension that clamps that stack together is released. Now the clutch discs must be allowed to freely slide apart out of their locked position. Remember, they ride along that clutch basket arm, so if you have grooves in it from where they sit 99% of the time (locked position, clutch engaged) then they are going to get hung up and cause you some issues.