Theoretically: how something works 'in theory' is basically like how something works 'on paper' (if you've heard that expression?) compared to how it works 'in practice'. As an example, the policy of asking to see customers' ID in a bar theoretically prevents or should prevent people who are under age being served alcohol. But in practice it doesn't really work because people either use fake IDs or borrow someone else's.
Constrained by: simply means 'restricted by', e.g. his ability to communicate was constrained by his limited knowledge of the language.
To which Parliament is accountable: this basically means that, because Parliament is made up of representatives who are elected by the public to represent them, those representatives can be held to account by the people who voted them in - which means that if the voters disapprove of what their representatives have done, they can choose not to elect them again. Think of it as the opposite of a dictator, who is not accountable to anyone because he can do what he wants and remain in power.
Economic, moral and political necessities: this part is quite vague and open to your interpretation, which makes it difficult to define. This part of the question is essentially asking you to describe which economic, moral and political factors Parliament might take into account when making law which would in practice stop it from simply making ANY law it wishes.
I hope that was helpful.