The expression of genes based on environment.
Basically we all get issued our genes at conception, but the way that they are expressed (in the simplest terms: turned on or off) is based on our experiences.
Epigenetic factors are therefore ‘actionable’: we can do something about them~. They include such things as diet, exercise, toxins, sleep & stress. Pretty much everything we are exposed to, in fact.
This is a cute introductory video on epigenetics: