Occurrences are a type of random event where something occurs. Bet that was a surprise. They can effect one, some or all the characters and they may last for a number of rounds. Normally, when a character experiences a number of events at one time, they are resolved in the order they are encountered. However, occurrences are always resolved first and only the first one is encountered. If there's more than one, the others occurrences beyond the first are ignored.
Occurrences come in 3 types: General, Natural & Personal
What makes natural occurrences different from general occurrences is that they are usually based on a weather change or natural phenomenon of some sort. Because of that, any natural occurrence that is encountered while another natural occurrence is 'in effect' is ignored. Also, if a character has druidic talents, they can ignore the effects of a natural occurrence.
A personal occurrence is simply an event that only affects the character that encounters it.