It depends on where you are. If you are in the EU or EEA countries, you can Roam like at home using a goodybag for most calls, texts and data, for up to 60 days in every 120, as long as you've used your account in the UK for a week or two first.
If you are anywhere alse in the world, you will need airtime credit to make calls (often even to receive calls as well), and send texts and use data, and it can be very expensive. Where possible, I would reccomend checking if you can get a local Pay As You Go SIM card.
For more information, see the giffgaff pages on Roaming, here: