In any stable climate time period (decade, century, millennium), there is some climate variability. In some years, the average temperature is hotter or colder than others but, on average, extremely cold and hot years occur infrequently. Most often, temperatures fall somewhere in the middle.
An increase of a few degrees in the "average temperature" shifts the entire curve to the right. This means less cold weather. It also means more years that have the former "extreme" hot weather. It also means the affected area will likely set new records of extreme heat.
A shift of only a few degrees in the average global temperature will likely result in more frequent and extreme heat waves.