help with subscribers to site

Hi, we have built new site and are newbies to WordPress, and suddenly there appears a new subscriber. I have no idea who this person is or what they are actually wanting. I’m not actually sure how they even subscribed as we do not have ‘subscribe2’ or any other plugin installed. We do not have a contact form on our site, only an email. I have searched the documentation but need to know quickly what to do…shall I email the person and ask them to send me info about themselves and what they want. or can anyone subscribe and suddenly be on your users list? I will continue checking out the codex on this… I know it’s basic, but….thanks in advance for any reply