For all entries

Hi All

a lines from certification books

"For all entries works with the database in a quantity oriented manner. Initially, all data is collected in an internal table.

You must evaluate the value of sy-subrc after each select statement. If the result of the first array fetch returns no data, the following select statement will retrieve all entries from the databse."

plz tell me why we need to check value of sy-subrc if we are using for all entries??

whatz an array fetch??

Thanks