Tap and swipe are two common gestures that allow the user to perform primary actions on their mobile devices. It's like a keyboard shortcut, but instead, you drag the message to the side.
And you can use the same gestures in eM Client on your touch device. This feature does not work on touchpads, it's only for touch screens right now.

You can use left and right swipe gestures in the Message list and in Notes.
The actions for both right and left swipe can be set in Menu > Settings > General > Swipe (Windows) | eM Client > Preferences > General >Swipe (macOS).

On Mac you need to enable 'Swipe between pages' in your System Preferences > Trackpad settings to make it work in eM Client.