The user the web server should run as. This means that the web server can only read and write those files which that user has right to read or write, respectively. Therefore this should be set to a user with very low priviliges, e.g. nobody.
Even better 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, as the user nobody
should not have any rights to write files anywhere except in
/tmp
.