What do the numbers 1-7 stand for in an electron configuration?

Fatal error: Uncaught TypeError: array_slice(): Argument #1 ($array) must be of type array, bool given in /var/www/web/proprofsdiscuss.com/SSI/function_live.php:661 Stack trace: #0 /var/www/web/proprofsdiscuss.com/SSI/function_live.php(661): array_slice() #1 /var/www/web/proprofsdiscuss.com/SSI/function_live.php(3592): related_popular_ques() #2 /var/www/web/proprofsdiscuss.com/question.php(1147): get_related_qsn() #3 {main} thrown in /var/www/web/proprofsdiscuss.com/SSI/function_live.php on line 661