php - Fatal error: Allowed memory size of 50331648 bytes 这什么意思

Fatal error: Allowed memory size of 50331648 bytes exhausted (tried to allocate 78 bytes) in /home/u500028/ews/workspace/php/appcode/webroot/htdocs/ThinkPHP/Library/Think/Db/Driver.class.php on line 289

这句话什么意思