Overall, however, the effects on euro area HICP inflation are expected to be moderate. Once the impact of the pandemic recedes, the unwinding of the high level of slack, supported by accommodative fiscal and monetary policies, will contribute to a gradual increase in underlying inflation over the medium term.
Tentatively, you can consider that it means something like “a system of rules that form the basis of policy choice” or “a basic framework of thinking.” You can simply say “the rules of a game.” A regime change in a policy means that the system of rules that forms the basis of policy choice changes.