What is the meaning of this test expression []?

I’ve seen someone use command: ps -ef | grep [h]ttpd and Output is: apache 25125 31006 0 21:54 ? 00:00:00 /usr/sbin/httpd apache 26869 31006 0 22:04 ? 00:00:00 /usr/sbin/httpd apache 27349 31006 0 22:07 ? 00:00:00 /usr/sbin/httpd apache 27696 31006 0 22:09 ? 00:00:00 /usr/sbin/httpd apache 28534 31006 0 22:14 ? 00:00:00 /usr/sbin/httpd root 31006 … Read more