WordPress comment cookie.
Note: In version 3.4 of WordPress the code that creates this cookie is produced by pluggable code. Therefore this cookie may only be produced when WordPress comments are being handled by the default WordPress code.
Cookies are only used if an ‘anonymous’ user leaves a comment – WP then puts your name, your email address ,and subscribe preference in three separate cookies. They just to store those details to save the user having to enter them again for a subsequent comment.