MTHFR stands for Methylenetetrahydrofolate reductase. As the name indicates, it is a process involved with the metabolism of folate (Vitamin B9). Many know folate (or folic acid) as an important vitamin that pregnant women should take.
MTHFR is doing the conversion of Folic acid to it's active form. There are certain genes that regulate how good this process works. The two big ones are MTHFR C677T and MTHFR A1298C. If you happen to have a mutation on one or two of those, you likely have an increased risk of certain diseases due to your reduced capacity of converting folic acid into it's active form.
There are tons of co-factors involved too, so a basic protocol, if you have a mutation in these genes, would include taking the active form of folic acid, the active form of B12, and often B1, B2 and some other supplements.
Dr. Ben Lynch from Seeking Health: www.mthfr.net
...to be continued....