By default, all filters are calculated independently. That means that for each filter Tableau runs through the entire data source and figures out all the rows that pass each filter individually and then returns the union of the results. For example, say you’ve filtered a view to only show the Western region and you’ve added another filter to only show sales from the first quarter of 2009. Tableau first looks at all the records and pulls out all the ones from the Western region. Then it goes back through all of the records again and pulls out all sales from Q1 of 2009. Finally the result is the union of both these independent filters.
Sometimes you’ll want to first run one filter and then run other filters just on the results of the first one. These types of filters are called Context Filters. When a filter is added to the context it is the independent filter and all other filters are only computed just on the results of the context.
NOTE: If your table has complex aggregations and is more than a simple view table, you should be very careful using this option!
For More Details :
Go to Filters Area → Right Click on the filter field → Add to Context.