is the file that you alter to change configurations and options with your inbuilt Apache Web Server. You can change things like the root placement of your web site (from /Library/Webserver/ on your system drive to another place such as another hard drive/partition volumes (more convenient in my opinion)), alter the error page messages, loads of things.
The best thing is, the file is so well documented within, that it really isn't that hard to deal with, as long as you have a definate idea what you want to do.
If you are interested in looking at it type/paste this into your terminal command line:
pico /etc/httpd/httpd.conf
This will open the file in Pico, the Terminal's own text editor application (you'll have to get used to using that as well, although you can use BBEdit or something, you just have to make the file available to it (non-invisible)).
Don't change anything unless your confident (or if your curious, back the file up, and remember/write down what you changed). When you do change something, you have to restart the Apache server with the command:
sudo apachectl restart
If you get an error after this (normally a message stating it was unable to restart), you know that whatever you changed was at fault (so go back and reset, or alter you httpd.conf file)