The autonomic nervous system is traditionally described as consisting of two mutually exclusive and balanced branches - the sympathetic and the parasympathetic systems. You probably know these better as the "fight or flight" mode (sympathetic system), and the “rest and digest” mode (parasympathetic system).
The sympathetic arousal system kicks into gear when we perceive threats. When that happens our blood pressure increases, our muscles tense, our heart beats faster, and digestion slows down.
The parasympathetic system is the opposite. On a lazy Sunday, curled up on the couch with a good book, you're probably in parasympathetic mode. In this situation you're "resting and digesting"; blood pressure decreases, pulse rate slows and digestion is in full swing.
These days many people are plagued by an inability to distinguish a "safe" environment from a "threat" environment. This causes these threat systems to be constantly engaged, even without us realizing it. It's like being constantly “jacked up,” but for no good reason. Being in constant sympathetic mode is not healthy for our organism. We simply weren't designed to operate this way.
What makes you feel safe? How often during the day do you feel like you're in a threat-free environment? When was the last time you actually relaxed?