I don't understand error log

Hello:
I don't understand this message of smartctl -l error /dev/sda:


Error 1 occurred at disk power-on lifetime: 21 hours (0 days + 21 
hours)
   When the command that caused the error occurred, the device was in an 
unknown state.

   After command completion occurred, registers were:
   ER ST SC SN CL CH DH
   -- -- -- -- -- -- --
   10 51 01 7b df 4c ab  Error: IDNF at LBA = 0x0b4cdf7b = 189587323

   Commands leading to the command that caused the error were:
   CR FR SC SN CL CH DH DC   Powered_Up_Time  Command/Feature_Name
   -- -- -- -- -- -- -- --  ----------------  --------------------
   ca 00 01 7b df 4c ab 08      00:04:44.912  WRITE DMA
   ca 00 01 40 00 00 a0 08      00:04:44.912  WRITE DMA
   c8 00 11 16 08 04 a2 08      00:04:44.912  READ DMA
   c8 00 56 bf 08 04 a1 08      00:04:44.912  READ DMA
   c8 00 56 69 08 04 a1 08      00:04:44.912  READ DMA



Thanks

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