What's an email filter?
All incoming email messages don't have to automatically go into your Inbox. An email filter lets you tell your software to handle some messages differently - for example, delete certain messages before you see them, automatically route them to a different mail folder, etc. A filter has two basic parts: The first is the criteria - this tells the software which messages you want filtered (for example, messages from a cerain person, messages containing certain words, etc.). The second part is the action - this tells the software what you want to do with messages that meet the criteria (ex. delete them).
One popular use for filters is to delete junk mail before you see it - for example, if you're getting a lot of messages trying to sell you low-cost mortgages, you can have a filter look for the words "low-cost" and "mortgage" in the body of your messages, and delete any messages containing those words. Be careful though - if your brother writes you all excited about the new low-cost mortgage he just took out, his message will be deleted, too!