A Spotify Gift Card is a physical card that you can buy from some retailers, it allows you or your friend to redeem Premium time on a Spotify account. You'll usually find them at the counters of the selected stores, and they're available in most larger markets.
If you've obtained a Gift Card, go to Spotify.com/redeem to redeem the code found on the backside of the card in exchange for Premium months. Just make sure you're logged in on the right account when redeeming it.
Here's some useful information about Gift Cards:
There is an 18-months limit to the amount of Premium months you can stack up at one time.
They are valid for 12 months from date of purchase.
The country a Gift Card was purchased in must match the country setting of the Spotify account for it to be redeemed. A Gift Card bought in the US can not be redeemed on an account based in the UK.
Spotify sells Gift Cards through Amazon.com with the retailer name 'Spotify' - cards sold by other retailers on Amazon are not authorised.