What is event handling in SSIS?

Like many other programming languages, SSIS and its components raise different events during the execution of the code. You can write an even handler to capture the event and handle it in a few different ways. 

For example consider you have a data flow task and before execution of this data flow task you want to make some environmental changes such as creating a table to write data into, deleting/truncating a table you want to write, etc. Along the same lines, after execution of the data flow task you want to cleanup some staging tables. 

In this circumstance you can write an event handler for the OnPreExcute event of the data flow task which gets executed before the actual execution of the data flow. 

Similar to that you can also write an event handler for OnPostExecute event of the data flow task which gets executed after the execution of the actual data flow task.

 Please note, not all the tasks raise the same events as others. There might be some specific events related to a specific task that you can use with one object and not with others.