Shopify connector user, or Dotdigital platform-only user?
If so, abandoned cart functionality is configured and implemented differently - and the information below will not apply. Please instead read the Shopify connector or platform-only (abandoned carts module) articles.
Overview
Enabling the easy email capture functionality will allow you to capture your site's user's email addresses when they haven't fully completed your checkout process or newsletter sign up process. This will mean you'll still be able to send them an email, re-engage, and potentially get them to complete their order or newsletter subscription.
It is accessed by going to System> Configuration > Dotdigital > Configuration > Abandoned Carts.
Easy email capture (checkout)
Enabling this updates Magento with a user's email address at the point of them leaving the email address field of your checkout process. This is particularly useful if you have a single page checkout process (meaning you don't have the ability to capture an email address via submission on a previously completed page). If a single page checkout is abandoned, then you're still able to send an abandoned cart email thanks to the easy email capture functionality.
Easy email capture (newsletter)
Enabling this updates Magento with a user's email address at the point of them leaving the email address field of your newsletter sign up process. This is particularly useful if you have a single page newsletter sign up process (meaning you don't have the ability to capture an email address via submission on a previously completed page). If a single page sign up process is abandoned, then you're still able to send an email thanks to this easy email capture functionality.
Please comment underneath with any questions you may have.
Magento 2.0
The above also applies for Magento 2.0, with the only difference being that the 'Easy Email Capture' sections are found by going to Stores > Configuration > Dotdigital > Configuration > Abandoned Carts.