Category D prisons (as open prisons are officially called) have minimal security. For example, at HMP Hollesley Bay, prisoners might be working on the farmland which surrounds the prison buildings. It would be very easy for someone to run off across the fields, to freedom.
However, Category D prisoners are usually people nearing the end of their sentences. It makes far more sense for them to spend a few final months complying with prison regulations than to run off and spend the rest of their life 'on the run'.
Prisoners can only be allocated to Category D status (and, consequently held in open prisons) if they're assessed as representing no threat to the public.
It's common practice, throughout all prisons, for long-term prisoners to be allowed out, on a daily basis, to gain work experience in the community. However, this probably occurs even more often with open prisons because the prisoners have already been assessed as not presenting any risk to the public and because open prisons have better systems in place to operate such policies. (e.g. they have transport systems which can take prisoners to the local bus or rail station each morning, and back again in the evenings, so that they can get to and from work).
Chris