The view filter window lets you restrict the list of senders to those that match whatever you type in the view filter window. This does not actually change the lists in any manner --- it is simply a mechanism to let you find particular senders more easily. It's particularly useful when you have a large number of senders in any of your lists.

Typing a sequence of characters like "abc" will show you all senders that contain "abc" in their name or email address. You can also use the * symbol as a wild character to refine your filter. For example, entering "abc*" into the filter field will list only senders whose email address or name starts with "abc".