The user the web server should run as. This means that the Web server can only read and write those files which that a user has rights to read or write, respectively. Therefore, this should be set to a user with very low privileges, for example, nobody.
An even better idea is to create a user specifically for the Web server.
This is very important, if you want to let the Web server write files in
some directory, since the user "nobody" should not have privileges to
write files anywhere except in /tmp.