It's anything that is a different type of prescribing to a typical medical prescription for drugs.
So for example GPs can make referrals to access weight watchers or to the local gym, or perhaps an outreach service or voluntary service 'on prescription'
In theory it's an attempt to look at the person holistically rather than just medically and treat causes rather than just symptoms. In reality it's got to be backed up by good community services so can be a bit of a postcode lottery.