Yahoo! Mail Filters are AND Filters
If you have a filter with the criteria From header contains sender@example.com and Subject contains About Email, only messages that have both "sender@example.com" in the From: line and "About Email" in the Subject: line will be moved to the specified folder.
A message from from@example.com with the subject "It's raining" won't trigger the filter, but one from sender@example.com with the subject "Re: About Email" will.
