Fluorine is a chemical element (atomic number 9). It's a gas at normal temperatures and pressures. It's the most reactive element in the periodic table and combines with every other element to form compounds. Under some conditions, it can even form compounds with the otherwise stable noble gases.
Because of its reactive nature, it's very toxic and can easily burn any tissue it comes into contact with, particularly the mucous membranes as it reacts violently with water. Although it's rarely found in its gaseous state, it is common in the earth's crust and can be extracted from several minerals. Because of its unstable nature, it's usually processed by combining it with other elements to form more stable compounds, most commonly hydrogen fluoride.
Its primary medical use is in dentistry as fluoride compounds inhibit the growth of bacteria and combine with growing teeth to make them more resistant to dental caries. Many municipal water systems add fluorine compounds to their drinking water for this purpose. Fluorine compounds are also found in about 20% of pharmaceuticals either as part of the active ingredient, or as a buffer to prevent the immediate uptake of the drug, allowing the therapeutic effect to be extended over time. Fluorine compounds also assist drugs in passing through cell membranes. Fluorine compounds are also used in many modern anesthetics.
One of the uses being developed for carbon-fluorine compounds is as artificial blood as these compounds are non-reactive (they won't harm blood vessels or other tissues) and can carry far more oxygen and carbon dioxide in solution than blood can.