A fever is a body temperature of 38C or above in children, or above 37.5C in adults. Most commonly, it is caused by an infection, and it is not normally anything to be too concerned about. However, a severe fever may be a sign of something more serious that needs urgent medical attention.
In most cases, treating the underlying issue will usually cause the fever symptoms to disappear after a couple of days. You can check for a high temperature at home using a thermometer, which you can buy from many high street stores or pharmacies.