The app is divided into "cards". When viewing a list or a task, there is a card for each; these will stack atop one another. It's then possible to slide these cards left and right to continue navigating.

Just try it out by tapping and sliding around the app!

Note: When trying to navigate from a task list back to the side menu, swipe from the left edge of the screen to the right. The left edge of the screen is reserved for this motion, even if there's a task underneath.