'Support mechanism' is a psychological term for something which you depend on to keep you healthy and sane in your day to day life. Support mechanisms can provide stability, or act as outlets for pent up emotions.
The best support mechanisms are other people. Ideally, these relationships are reciprocal, because you support them as well. The normal examples are your friends, SO, family, etc.
Good support mechanisms can turn nasty. Hitting your wife may be one way of (temporarily) releasing stress, but it's Bad and Wrong. Any relationship can turn abusive if you try hard enough.
Any part of your life which serves to distract you from the rest of it serves as a support mechanism. For example, spending time with your friends can help you to forget about work for a while.
It is possible to be over-dependant on any support mechanism. But any level of dependency on some things are just bad. These tend to be one-sided relationships with inanimate objects:
Regardless of the legality of the chemical, dependency on these things is bad. Good support mechanisms do not alter your perception of reality - they merely shift your attention within it.
Various media also serve as a conduit for support mechanisms - television, radio, newspaper, the Internet. There was a time when reading the evening paper was a form of relaxation. There are people who watch hours of television at a time, which is one form of relaxing.
Having good support mechanisms is an important part of your life. Notice the plural: having just one is a bad thing, even if that support mechanism is something socially acceptable like religion. A lack of support mechanisms tends to lead to large amounts of stress, decreasing self worth, and lower self-esteem. It eventually leads to self-destruction.
1: There are of course special circumstances. I am referring to addiction, not prescription.