Each inode contains a count of the number of data blocks that it references. The number of actual data blocks is the sum of the allocated data blocks and the indirect blocks. The fsck command computes the number of data blocks and compares that block count against the number of blocks that the inode claims. If an inode contains an incorrect count, the fsck command prompts you to fix it.
Each inode contains a 64-bit size field. This field shows the number of characters (data bytes) in the file associated with the inode. A rough check of the consistency of the size field of an inode uses the number of characters shown in the size field to calculate how many blocks should be associated with the inode, and then compares that number to the actual number of blocks claimed by the inode.