Methylation is a biochemical process (involving chemistry of compounds and chemical reactions) involving the transfer of a methyl group (one carbon atom connected to 3 hydrogen atoms, or -CH3) to another chemical compound, for activation of the resulting compound or to cause some action.
Of more relevance to health in general, methylation is also used for gene expression - methylation turns on or off the genes which can affect your health. This is called epigenetics - how your environmental factors turn on or off genes, without affecting the DNA underneath. For example, stress, your diet (and deficiencies), chemicals and toxins, lifestyle factors, sleep, etc, will either turn on or off genes to improve your health or to cause symptoms.
With regard to Pyrrole Disorder, there is a belief that people with this condition are categorised into being an overmethylator or undermethylator, to explain how well (or not) the many body methylation processes are occuring. Some practitioners use a questionnaire of symptoms to determine if someone is one or the other, but I've never seen someone who has just symptoms of under vs over, so I question how true this classification really is. Other practitioners test histamine levels as a marker of over vs under, and others use different testing - there is no gold standard. Some genetic testing of genes such as MTHFR and others are sometimes used to check for methylation status too. Vitamin B12 and folate in your diet are methylation factors and do affect methylation, but should be taken into account for assessing methylation status, but these are typically not used for some strange reason. Many other nutrients are needed for methylation, or their deficiencies can affect methylation too.
The thyroid function of controlling your metabolic rate will also affect methylation, and typically isn't taken into account either! I seriously question this methylation classification and I do not feel it is relevant for health in general or for pyrrole disorder. I do not believe that EVERY chemical process of methylation in the body is somehow running under or over! Some will likely be under and some being overmethylating and this will regularly change, because of many factors, hence why people with pyrroles have symptoms of both!
In undermethylated people, the problem is usually due to a lack of methyl group production in methylation pathways from the abovementioned causes.
It actually isn't possible to be an overmethylator as methyl groups and donors are available, but the cells or processes cannot actually use them, leading to high methyl levels hence the term "overmethylation", but causing a functional deficiency of methylation and LOW methylation symptoms. Hence "overmethylation" is just UNDERmethylation but having high availability of methyl groups.
Hence don't worry about your alleged methylation status!