Context

This indicates where in the server's configuration files the directive is legal. It's a comma-separated list of one or more of the following values:

server config

This means that the directive may be used in the server configuration files (e.g., httpd.conf, srm.conf, and access.conf), but not within any <VirtualHost> or <Directory> containers. It is not allowed in .htaccess files at all.

virtual host

This context means that the directive may appear inside <VirtualHost> containers in the server configuration files.

directory

A directive marked as being valid in this context may be used inside <Directory>, <Location>, and <Files> containers in the server configuration files, subject to the restrictions outlined in How Directory, Location and Files sections work.

.htaccess

If a directive is valid in this context, it means that it can appear inside per-directory .htaccess files. It may not be processed, though depending upon the overrides currently active.

The directive is only allowed within the designated context; if you try to use it elsewhere, you'll get a configuration error that will either prevent the server from handling requests in that context correctly, or will keep the server from operating at all -- i.e., the server won't even start.

The valid locations for the directive are actually the result of a Boolean OR of all of the listed contexts. In other words, a directive that is marked as being valid in "server config, .htaccess" can be used in the httpd.conf file and in .htaccess files, but not within any <Directory> or <VirtualHost> containers.