ICS event correlation

W​‌‍‍‍‌‍‍‌‍‌‌‍‍‍‌‍‌‌‌‍​hat is event correlation and​‌‍‍‍‌‍‍‌‍‌‌‍‍‍‌‍‌‌‌‍​ how can it be used in ICS’s​‌‍‍‍‌‍‍‌‍‌‌‍‍‍‌‍‌‌‌‍​?