What is purpose of food supplement such as ensure or boost? who should take it and is it food or a prescription? what is in these supplements?

Caloric supplements: Ensure & Boost are two popular brands of caloric supplements. They are intended as meal replacements for those who can't/won't eat enough. They are non-prescription foods. Check their individual labels for dietary ingredients as there are many versions of each.

Non-Optimal Options: for people who will not eat enough food to stay alive without them, especially those with problems taking in more solid foods. Despite the ads, they are essentially never a great/best option, are OTC & not required to prove benefit/downsides per DSHEA 1994 (Act promoted in US Congress to exempt OTC by this ad: https://goo.gl/nb8LtC paid for by OTC marketers, widely played to public on TV 1993-94).