The __cfduid cookie is used to override any security restrictions based on the IP address the visitor is coming from. For example, if the visitor is in a coffee shop where there are a bunch of infected machines, but the visitor's machine is known trusted, then the cookie can override the security setting. It does not correspond to any userid in the web application, nor does the cookie store any personally identifiable information.
Note: This cookie is strictly necessary for site security operations and can't be turned off.