​To "methylate" means to add a methyl group to another molecule.  This is how we create a new molecule (for example hormones, proteins, DNA, etc.).

It is important to understand that molecules do not just bop into each other.  These are very regulated reactions.  
Methylation is such a basic, frequent and important reaction in every cell, that the body created a type of "docking station" called SAMe.  SAMe binds in very specific ways to other molecules to transfer its extra methyl group.

Again, because this is such a common and important process, the body created a "Methylation Cycle" which is a "mini factory" to produce a lot of SAMe, to have it ready for the reactions that constantly take place in the body.

Some of the major reactions that need methylation "factors" include: 1) elimination of toxins; 2) production and breakdown of neurotransmitters; 3) production of energy and cell membranes; 4) regulation of gene expression.

In some people, this "methylation cycle" (the factory for SAMe and other methylation factors) does not work properly.  Some of the symptoms (of undermethylation) include:  depression and internal anxiety, difficulties focusing (or opposite, a little OCD), headaches or feeling "toxic", allergy-like symptoms (lots of histamine), fatigue, hormonal problems and often a family history of cancer or substance use (self-medication).

Important - I do not recommend taking SAMe by itself.  It is best used as part of a plan that treats the entire methylation cycle and the entire person.  Please seek professional advice.