If someone tells you they suffer from anxiety, their first reaction is usually to shrug it off and act like its not a big deal. Most people don't believe that anxiety is an actual mental illness; they think its the person's fault for stressing out so much and they should just "calm down." But sometimes the ability of calming yourself down is not a luxury that people with anxiety can afford.
Anxiety disorder is a real mental illness and it is recognized medically as such. There are many people that suffer day to day with anxiety and try to make their illness functional. You may not notice them at first but you can usually tell by the way they tap their fingers constantly, brush their hair back a million times and/or avoid making direct eye contact with others. Their is still stigma on many mental illnesses today in our society so many people are choosing not to talk about it for fear that their feelings wont be recognized.
Personally I suffer from anxiety and fortunately for me it has gotten better throughout the years. But some people would never know I had anxiety since I don't talk about it or show signs of being overly anxious in public. Many people with anxiety will cancel plans at the last minute just because of the stress off going out. Their mind works so fast that they are thinking about every outcome of the night and how it could go horribly so the easiest option is just to cancel. Another sign of anxiety is not feeling comfortable in one place, you feel like you have to keep moving. All of these are normal things that people do when they are faced with this mental illness.
If you have a family member, loved one or friend that suffers from anxiety, be there for them and listen. Don't devalue their feelings just because they differ from your own.