What does replay-to mean in an email? Does that mean who the people need to reply to if they have any questions, and I would put an email address?

You can have a
From Address: ostensibly who this came from (you@you.com)
Reply To: is an optional address in case someone hits reply. perhaps your From is not a real box, or is someone who isn't going to look at emails, so you route replies to a proxy box or someone else: marketing@you.com or lauren@you.com.

Usually you will have both From/Reply To as the same email, but sometimes you might not want to.

Simply: Reply-To is the mailbox to which you want human-generated (i.e. manually composed) replies to be sent.

Technically speaking there is no need for a Reply-To if the From is the same address. The From address will be used in the absence of a Reply-To.  However, these days most people set both even if they're the same.

You'd have a different From and Reply-To if you want the From to reveal a human owner of the message that someone can deliberately copy-and-paste into a new message if they insist, while the Reply-To is an mailbox that doesn't actually accept messages (they are deleted or rejected).  This setup makes it less likely you'll get useless replies like "unsubscribe me" instead of people going to your web-based unsubscribe form.  Typically, tip people off that the Reply-To isn't ever to read a reply by calling it <please-do-not-reply@example.com>, <no-reply@example.com> or similar.  Really, if you don't call it something obvious like that you are implying that a human will read replies to the message, and it is unethical to then discard replies.
 

... interesting that Josh and I came at this from opposite angles.  He's of course totally right that you could have a From that doesn't accept messages and a Reply-To that does, so in that case you're putting the Reply-To "in front" to make sure a human reads those.

Either way, you should be able to manage your marketing process so that the Reply-To and From are the same address.  Make the decision now as to how you are going to process human responses to your marketing e-mails and then choose an address appropriately.  For the record, we do not use a <do-not-reply...> but a real sales group mailbox that is checked... well, regularly enough!