Enter your Fabletics e-gift card number at checkout on Fabletics.com, in the same field where you'd enter a promo code. If your order costs less than the card's value, the remaining balance is added to your account as store credit, which you can see under Membership Information in My Account.
E-gift cards can only be redeemed at online checkout. Once the balance is store credit, you can also use it in Fabletics stores.