The medical supplies are items that you can use to heal yourself. They can especially be useful in hardcore servers, where you cannot regenerate health. There are currently 4 Medical Supplies, listed below.
The Rag is a Level I Medical Supply. Right clicking it will restore 2 Hearts of your health, and will extinguish you if you are on fire.
The Bandage is a Level II Medical Supply. It is basically a Rag, but it restores 4 Hearts of your health (2 additional hearts). Just like the Rag, it will extinguish you if you are on fire as well.
The Splint is a Level I Medical Supply. It restores 2 Hearts of your health.
The Vitamins are the only Level III Medical Supply. Consuming it will restore 4 Hearts of your health, and cure you from any poison, wither and/or radiation effects.