Google Tasks is an ancient (in technological years) service that somehow manages to look even older than it actually is. If you didn't know about Tasks to begin with, we wouldn't blame you, as it's almost if Google itself had completely forgotten about it and just recently decided to revive it.
Fortunately, Google has finally realised that it needs a core "to-do" platform that converges all of your tasks and to-do lists from Gmail, Calendar, and other Google services, and allows you to easily manage them. Enter Tasks, the new Google app for Android and iOS that's being introduced alongside the redesign of Gmail for web.
The idea of Tasks is simple – it's an app where you can manually add and organize various tasks (or have them automatically imported from other services) that don't necessarily fit in your Calendar schedule. It's more of a day-to-day thing that caters to the routinely, and it's a tool that Google has been missing up until now. In stand-alone form, at least.
Aside from the mobile Tasks app, Google is also working on refreshing the ancient web version of the service.