It seems that it is an issue with reserved words in MySQL engine. You should use double quotes or backtick/backquotes with these words. For example:
$sql="INSERT INTO groups(`Desc`) VALUES('$_POST[desc]')";
Please refer to MySQL 5.7 Reference Manual 9.3 Reserved Words
] for more details.
Also consider to use parameters instead of using a $_POST or $_GET variables. For example:
$statement = $db->prepare("INSERT INTO groups(`Desc`) VALUES(:desc_column)");
$statement->execute(array('desc_column' => $_POST[desc]));