Your life is a complicated thing with many aspects to it. It is convenient to divide your life up into parts, which we can call "life-domains" so as to be able to talk about one part of your life at a time. We need to be able to do this because while some problems affect many aspects of your life (many "life-domains") at once, other problems are fairly domain specific. In such cases, we want to be able to identify just the parts of your life that are affected by your problem, so that we can give them special attention. Also, because different aspects of your life are often addressed best with different methods, it is helpful to be able to describe how even complex problems affect the different aspects of your life, so that you can address each aspect in turn. Breaking problems down in this manner helps you to solve them more easily.
We should give some examples of how problems can be complex, touching many aspects of your life at once, or simple, causing difficulty in only one or two life-domains. Depression and other illnesses tend to be complex problems, particularly when they have been going on for a long time. Depression can make it hard to stay employed, for instance. It can alienate you from friends and family. It can make you unwilling to exercise or to take care of yourself. Depression can cause all of these problems at once, making it a complex problem to address. Other problems are simpler in nature. You might have a dispute with a coworker, for example. Your work related problem may result in tension while at work, and some sleepless nights, but it is unlikely to be related to your ability to make friends in general, or to have a satisfying home life.