The check operation scans the drives for bad sectors and automatically repairs them. If it finds good sectors that contain bad data (the data in a sector does not agree with what the data from another disk indicates that it should be, for example the parity block + the other data blocks would cause us to think that this data block is incorrect), then no action is taken, but the event is logged (see below). This "do nothing" allows admins to inspect the data in the sector and the data that would be produced by rebuilding the sectors from redundant information and pick the correct data to keep.
Users may alternatively echo repair to /sys/block/md0/md/sync_action but this is ill-advised since if a mismatch in the data is encountered, it would be automatically updated to be consistent. The danger is that we really don't know whether it's the parity or the data block that's correct (or which data block in case of RAID1). It's luck-of-the-draw whether or not the operation gets the right data instead of the bad data.