Great question! It means that their email address is unemailable. There are a couple of reasons this can occur:

1. Most likely, the person unsubscribed to email from a link at the bottom of an email blast. This is unfortunate, but the person's preference must be respected.
2. If the address was included in email blasts and bounced several times, the system will mark the address as bad and then put a line through it.
3. A person with control panel access may have edited the person's profile and unchecked the box "receives email." This may be done if the person sends an email to you asking to be removed from your mailing list. 

To re-engage people who have opted out of email, you can try connecting with them one-on-one via another method. If you have his/her Twitter handle, you could ask for feedback via a Direct Message (DM). If you have their Facebook User ID, you could send them a message that way. And of course, calling them or sending a letter to their house (or even knocking on their door for a face-to-face conversation) are still effective ways of connecting with your organization's supporters. Remember to log these contacts within the person's user records, so that your people database has a complete history of your interaction with people in your community.