lets say, you need to add 2 more fields to a report... and if you are selecting those records from a table... so in turn you'll have to add those two fields in the SQL Query too.
Now Impact analysis should check what are the other components can get affected by this change... such as:-
1> What are the other programs using this dataset ?
2> If it is a copybook, associated programs would also need attention.
This is just a small example... basic idea is to enlist all entities associated with the component going to be changed.