In general the Halo effect refers to a positive view of something without merit. For example when movie stars endorse products, the consumer thinks the product is good or worth buying because the celebrity said so. The celebrity may not really know anything about the value of the product.
In another example people who are physically attractive are often judged as smarter or more capable than someone of average appearance.
In the medical world, sometimes patients rate the benefit of a treatment method higher than the actual results show. For example, patients with chronic neck pain from a whiplash injury may say they are very pleased with the final results even though they still don't have full neck motion and pain remains. If they get enough pain relief to relax the muscles and move the neck more freely they may say they are 100 percent satisfied.
When motion is only 50 percent improved and pain remains rated a five out of 10 then that satisfaction level doesn't match the results. This is one example of the halo effect. Medical researchers attribute a feeling of well-being in patients who have some but not total improvement as the halo effect.