Eva, Deplin is a trade-name for L-methylfolate.
What that means is that it will increase the levels of L-methylfolate in your body, which are probably low because of your MTHFR mutation.
I am sure that mrsD will be able to offer you more detailed advice than I can but in the meantime this general information might help you;
http://www.drugs.com/cdi/deplin.html.
As far as I can see Deplin has an excellent reputation and I really hope that it works for you.