A line doesnt refer to a physical line, it refers to each statement terminated by a semi-colon.
As for finding the error, obviously you would go to the line and take a look. The problem may lie on the line itself, or be the result of an earlier error that the interpreter thought was acceptable.
If the error is given on a line is that outside the number of lines the script has, then it could mean that somewhere or another the interpreter got confused by incorrect code.