Methylation

Methylation is when a methyl-group (some stuff $R$ attached to a $CH_3$) is added or replaces an atom in a larger molecule.

By Jü - Own work, Public Domain, https://commons.wikimedia.org/w/index.php?curid=10563908

Often, a methyl group replaces a hydrogen attached to a carbon. Since carbon can attach up to 4 hydrogens, the methylation process enables a 2-bit state in a biological circuit. A biological circuit can run the methylation process to store feedback about the environment, provided that there are still some hydrogen atoms (or compounds) to replace with a methyl group (or, conversely, that there is some methyl group to replace with the original hydrogen compound). Methylation can be one way of achieving sensory adaptation.

In DNA

Some RNA/DNA bases can be methylated, too. In fact, the difference between T and U bases is simply replacing one of the hydrogens with a methyl group.