Many Visa gift cards can be activated over the phone. How a gift card is activated is up to the card's issuer. Some are activated at the time of purchase, requiring no activation by the recipient.
Once a card is activated, it is important for the recipient to sign the back. This prevents others from using the card.
Once activated, the Visa gift card is ready to use. It can be used online, over the phone and in stores, the same way a debit or credit card is used. Once the recipient spends all the money on the card, more can be added or the card can be thrown away.