Anxiety is fear, which is usually less intense than outright terror, which is constant, and which stubbornly hangs on despite attempts at relief. It can occur for a variety of reasons including but not limited to having post traumatic stress or having some type of brain malfunction. There isn't always an explanation for its occurrence.
Anxiety can respond to medication, but can also sometimes be dealt with through meditative exercises. For a period of many months, I would experience an anxiety attack at the same time every day. I never found an explanation for it, but am able to deal with it through meditating and resting at that time of day.
Severe anxiety can be harder to deal with than you might expect. It is generally categorized as a much less severe disorder than psychosis or bipolar. (This is probably because anxiety doesn't usually cause odd and unexplained behavior and it doesn't strip the mind of the person's ability to survive in society. It is just very uncomfortable.) Many people in the "mainstream" population, who work and who essentially qualify as "normal people" suffer from chronic anxiety. People with anxiety do not get regarded as "crazy" as do those with schizophrenia or bipolar.
Despite the less severe nature of anxiety, it is sometimes a hard condition to treat because the medications most effective at treating it can become addictive. Clonazepam, for example, loses much of its effectiveness if a person remains at a constant dosage. Benzodiazepines, in general, require periodic increases in dosage in order for them to retain effectiveness. This is a part of the definition of addiction.
When I first became psychotic, about thirty years ago, I experienced anxiety that was so bad that all I could do was lie flat on my back. In my case, the anxiety is sometimes a precursor to another episode of psychosis. In other instances, it is merely another "perk" in my package deal of mental illness.
Anxiousness can be an unbearable emotion and it can sometimes be resistive to attempts at resolving it with meditation. In my case, anxiety often persists until I either take an anti-anxiety medication or else meditate at the top of my ability. Sometimes I am simply stuck experiencing it. It can be debilitating because it makes me immobilized. Doing something to distract myself including working or getting physical exercise seems to only increase the intensity of the discomfort. Attempts at being active tend to worsen the anxiety, and this type of multitasking can be excruciating. When I have a major attack of anxiety, the last thing I want to do is move.
Anxiety can be uncomfortable and sometimes unbearable even when one knows intellectually that nothing is wrong. As with many mental health problems, medication does not always neatly solve the symptom. Combining medication with mental exercises and with mere toleration seems to be a better approach.