If you're just asking what the term "short line" is, then yes, it's a line of input data that has a record length that is less than the longest requested record length by the INPUT statement.
The missover/stopover/truncover options determine what to do when that happens (when SAS attempts to read past a line delimiter, meaning a CR/LF/whichever or both are used by your OS/program).
Kiranv's linked paper explains them well, but in brief: STOPOVER says to treat as an error any time it attempts to read past a line delimiter. MISSOVER says to treat any variable that extends past the line delimiter as missing (including one that starts before it). TRUNCOVER is similar but does not treat a variable that starts before the line delimiter as missing, it just truncates it to the line delimiter. FLOWOVER basically ignores the line delimiter and keeps looking for more data after it (and is default).
