It means that your card is just that: free to send to your list! You can send a free card to up to 2,000 email addresses. Many free card designs will automatically populate with an envelope once you begin to customize them. You can remove the envelope by following the instructions listed here.
Adding a premium option that costs coins (like envelopes, backdrops, logos, custom stamps, and liners) will allow your sending limit to increase to 15,000 addresses.
For more information about pricing, see: What are Coins?