The <RequestMap> element is the root of the XML-based RequestMapper configuration (see the general documentation on this plugin for context). The root element is a globally applicable container that can apply settings to all requests.
A special requirement of this root element is that it MUST contain an applicationId attribute with a value of "default", which in turn matches the required id attribute of the outer-most <ApplicationDefaults> element in the shibboleth2.xml file.
Apart from establishing default settings, its purpose is to act as a container for the rules you specify regarding virtual host, path, and/or query string matching to apply settings. The tree structure of the XML essentially maps to the tree structure of the URL space of your web server.
Matches requests with a URL prefix matching a regular expression. The prefix contains everyting up until the slash following the hostname or port.
The matching process is peformed by examining <Host> elements in order, and then checking any <HostRegex> elements in order, in separate passes (see the HowTo).
A somewhat non-trivial example shows a pair of virtual hosts with one bound to an <ApplicationOverride> called admin. The path "/secure" on the primary host is protected automatically, while the entire admin host is protected along with access control rule embedded in the configuration.