Receiving an invitation is usually considered a privilege. It means you are included in a group carefully selected by the host of the event. Lucky you!
However, with invitations comes a small amount of responsibility. The host sent it in order to know who might attend. So when she writes RSVP at the bottom, you are obligated to let her know if you will attend.
RSVP stands for respondez s’il vous plait. In English, this means please respond. What this means for you, the recipient of the invitation, is that as soon as possible, you should let the host know whether or not you are attending.
In other words, she is expecting to hear from you within a time frame suitable to continue planning her event with ease. Many people believe a response is only necessary if they intend to make an appearance. Not so. RSVP applies to all invitees. You should respond to an RSVP in the same manner that the invitation was extended. If by mail, then send a note of acceptance or regrets. If by email, then send your response that way.
The main idea is to let your host know whether or not you are coming. The guest who appreciates the invitation, and responds accordingly in a timely manner, is sure to receive future invitations. This is the kind of guest every host dreams of!