Hormesis, in biological processes, is a phenomenon where a low dose of x produces an opposite effect as a high dose of x.
For example, low doses of stress energize you and increase your performance. Very high doses of stress paralyze you and decrease performance. Low stress on your muscles (working out 3 times a week) strengthens your muscles, while very high stress (working out 3 times a day) leads to injuries.
One may apply the same idea outside of biological processes: a low dose of your hobby (5h a week) may make you happy, while turning that hobby into a job (40h a week) may make you miserable. Seeing your friends once a week produces a different result than living with them 24/7.
Dose-response relationships, a related term, describes how something harmless can become harmful, given enough dose or exposure to it. To be more precise, there aren't necessarily "harmful" and "harmless" things, just harmful and harmless doses of things. It is the dose or exposure that matters.