ETOPS is all about FUEL. Making sure, at the planning stage, that you have enough fuel on board at all times, for the worst case scenario, (Critical Fuel Scenario).
For the A330 aircraft, this is an engine failure and pressurisation failure at the same time, occuring at the Critical Fuel Point (a point on the route where there is the least amount of fuel left on board, which is available to cover the Critical Fuel Scenario). This would require an immediate descent, followed by a diversion at FL 100 to a suitable airport.
Clearly, there is much more to the subject than the above. But hope this gives you a basic idea.
Happy flying..........
pjumbo