Official Description
6.9.1 On reading a number from a text file, after skipping preceding spaces and end-of-lines, it is an error if the rest of the sequence does not form a signed-number.
Simplified Description
In other words, it is an error if the program attempts to read an integer value or a real value from a text file but the next characters to be read from the text file can not form an integer value or real value. For example if a text file contains the following
zzzzzzzzz
and you try to read in an integer value or real value from this file it is an error.
Error Handling
This error is reported if I/O checking is enabled.