A save the date card or magnet is a simple message to inform you of when the wedding is supposed to take place. That's all there is to it! A save the date card will have on it your name and a message informing you of the wedding day. They're intended to give you the best possible notice of the wedding date, making it as easy as possible for you to ensure that you're able to attend.
Life is full of commitments - family, friends, work, hobbies, trips, holidays, and so much more. So a save the date card gives you advance notice to have a certain date booked in your calendar if you really want to attend the wedding. But, it's not actually an invite.
A save the date card doesn't necessarily mean that you're invited to the wedding - it simply means that you're on the proposed guest list. The final guest list can be subject to change, for example, if there are unexpected changes with the venue, meaning fewer people can come.
That said, couples don't just send save the date cards to anyone, so if you've got one it's a safe bet that you are going to be welcome at the wedding. If you haven't received a formal invitation by roughly a month before the wedding date, it pays to check with the couple. Sometimes mistakes are made, invites are lost, and other miscommunications can mean you didn't get your invite.