Folate is a vitamin, B9, needed by the body.  L Methyfolate is the active form of the vitamin. It helps to regulate the synthesis of serotonin, dopamine, and norepinephrine. 

A key regulatory enzyme, methylene tetrahydrofolate reductase (MTHFR), converts dietary folate to useable L methylfolate.

L methylfolate assists in the formation of a critical cofactor, known as tetrahydrobiopterin or BH4 which actives the enzymes that produce the neurotransmitters.

People at risk for low folate include abusers of alcohol, pregnant women, those suffering with an eating disorder, those with high homocysteine levels, and patients on lamictal or valproate which interfere with the conversion of dietary folate to L methylfolate. 
Patients on antidepressants that fail to respond, should consider augmentation with L methyfolate, as without it, the key neurotransmitters, which regulate mood, cannot be synthesized.