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. fsck computes the number of data blocks and compares that block count against the number of blocks the inode claims. If an inode contains an incorrect count, fsck 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 is done by using the number of characters shown in the size field to calculate how many blocks should be associated with the inode, and then comparing that to the actual number of blocks claimed by the inode.