The scope of a cookie can be limited to a certain part of the document tree within the web server. By defining a node within the document hierarchy, the cookie will only be sent to the web server when requesting a page that exists at that path or lower down in any sub-directories within it.
Unless you specify this value, the cookie will by default be available to any pages in the same directory as the page that created it or in pages lower down in sub-directories. These might be referred to as sibling or child pages. The path value is usually modified to be more inclusive than the default settings.