SharePoint Planner is a Filter Consumer - meaning that it will take information provided to it and filter the display accordingly. For example you could pass in a filter of "High" and get Planner to show only High priority tasks.

This page has details on how to setup web part connections.

You can configure how Planner uses these filters in this section of the toolpart.

You can specify which Operator to use (Equal, Not Equal, Contains, Begins with etc etc) and if multiple filters are passed if an AND or an OR operation should be used. These choices are the same that you would use when setting up the filter options for a View.

For most applications the default "Equals" and "Or" choices are appropriate.