Following an issue in Sifter allows you to be notified about certain changes made to that issue.
In the issues list or board view, you'll see an eye icon to the right of the issue row or card. That icon indicates whether or not you're following an issue. You can follow and unfollow issues by clicking the icon.
You can also filter your issues list or board by following, or not following (just in case you want to see what you're missing out on).
When viewing a particular issue you'll see a button in the top right that says either Follow or Following, and you can click that button to change the follow setting for that issue.
To manage your notification settings for issues you're following, go to Settings and then click the Notifications tab. You'll see several options for controlling when you receive an email about Sifter activity.