For a layman the word prophylactic is not needed for any significance other than general knowledge. In simple words prophylactic means a medication or treatment to prevent diseases occurring.
There are various kind of prophylactic measures including vaccinations. In general nature vaccination injections or oral doses are given. This is known to all. However on certain specific situation, say when there is a large scale flood or some similar natural calamity and damage, the population in that area are prone to catch certain diseases. In these situations certain drugs and medicines are administered to the whole population to prevent occurrence of diseases. That is prophylactic use.
Sometimes when a person suffers a particular disease there is also a probabaility of another disease occurring. Hence in such situations prophylactic drugs are used.
When a disease is running and the person is treated to manage and cure the disease, it is therapeutic treatment. Sometimes the same medicine can be used as therapeutic and prophylactic, but in different doses and potency. For example some vitamins and minerals may be used as daily supplements as prophylactic use. But the same vitamins and minerals may be used at higher doses or frequency when a person is affected with a disease.
To be a little more technical prophylactic treatment is preventive therapy and comes under preventive healthcare.