Update Row Counts will run a query to see the number of records for this table in the local instance as well as the number of records in the target instance. After the query has completed, the Local Record Count and Target Record Count fields will be updated with the respective number of rows found in each instance for this table.
This option allows you to see if the tables match up in terms of the number of records. If they do not, you can use the Compare Rows option for more detail on the discrepancies.