Once you receive the card you will need to activate it. Once active, you can use it anywhere you could use a MasterCard, both online and offline at stores, restaurants, etc.
However, it is still a 'prepaid card' so some websites may not accept it, and if you try withdrawing at an atm and you don't have enough money on the card, it may eat the card (i.e. not give it back, so you have to contact the atm bank if this happens). So always make sure that you have enough money before you withdraw from atm.
You may also withdraw to your bank account from Payoneer. You can add multiple bank accounts to a single Payoneer account.