Here is a link that kinda explains what a flare is. Still not a very clear explaination. I like explainations with examples.
One problem with rheumatoid arthritis is that the symptoms tend to come and go with no particular pattern. You may have 'flare-ups' – periods when the joints become more inflamed and painful. Sometimes this has an obvious cause – either physical, such as unaccustomed physical exertion or another illness, or emotional, such as bereavement. Usually, though, there is no obvious cause, however hard you think about possible triggers. This unpredictability is frustrating and makes it difficult to plan ahead. Rheumatoid arthritis can be a serious disease with a lot of symptoms. But most people, especially if they receive appropriate treatment, will have relatively few symptoms, and will be able to lead full, normal lives.
Rheumatoid arthritis signs and symptoms may vary in severity and may even come and go. Periods of increased disease activity — called flare-ups or flares — alternate with periods of relative remission, during which the swelling, pain, difficulty sleeping, and weakness fade or disappear.
Copyright ArthritisInsight.com