Filters let Apps mail perform certain actions on your messages based on criteria you provide. 

Want to apply a star to each of the 3000 messages from the school principal all at once? Here's how you would do it without filters:

1. Select the Label name - in this case it could be a label you have for messages from the school principal (or, your Inbox, or Sent Mail, etc) containing the messages you'd like to select.
2. Click the Select: All link above your messages.
3. Click the link that says Select all [number] conversations in [current view].
4. Select the action you'd like to take from the More Actions... drop-down menu.

That's easy enough for conversations that are already categorized where you want them, but what about the 200 messages scattered throughout your inbox related to Unit Test planning you sent or received over the past month? No problem. Just create a new filter so that Apps mail can automatically find the messages, by following these steps:

1. Click Settings (at the top-right of any Apps mail page).
2. Click Filters
3. Click Create a filter (at the bottom of your filter list)
4. Enter your filter criteria, for example, 'subject contains: Unit test'.
5. Click Test Search to see which messages match your filter terms. You can update your criteria and run another test search – maybe now you'd like the filter to only contain messages from specific teachers about the Unit Test planning.  Add the email addresses of other teachers in the 'sender' field or specify , or click Next Step.
6. Select one or more actions from the list. (Fun fact: these actions will take place in the order they're listed -- for example, you could choose to Forward matching messages to a specific email address, then Delete the messages).
7. Presuming you'd like to apply this filter to the messages you already have (rather than only to new messages), select the 'Also apply filter to [X] conversations below' checkbox.
8. Click Create Filter.

You can also create a filter based on characteristics of the message you're reading. For example, if you receive emails from a school mailing list (e.g. math-teachers@myschool.org), you can easily filter messages like those. Or if you receive several emails with the same subject, you can also start with an email message to create a filter.

To create a filter from within a message you're currently reading, just follow these steps:

1. Click the More Actions button at the top of the message.
2. Select Filter messages like this.
3. Enter your filter criteria in the appropriate field(s). Apps mail will try to automatically add the criteria based on the message you were reading, such as the mailing list name or sender email address.
4. Click Next Step and then select the actions you would like Apps mail to perform on these message - for example it could add the 'Math Teachers' label and archive it so the message is moved out of the inbox.
5. You can apply the filter to messages you already have by clicking the 'Also apply filter to [X] conversations below' checkbox.
6. Click Create Filter.