Tasks are all the things you need to do in order to get rewards for it. By selecting the to do list on the upper left screen edge you get an overview of all the tasks that are open or that were finished but the reward was not yet collected. If you don't know how to finish a task, you can select the "Show" button and a red arrow will lead you to the right place to do it.

In the queues you will get an overview of all the ongoing tasks. There, recovery times of units and hideouts as well as upgrading of provinces and fortifications are also listed as tasks, even though you will not get rewards for those.