An internal task is a task related to your company or colleagues, and not associated with a client. Examples of an internal task could include setting up a staff meeting, reserving space at a conference, making a call by COB, ordering office supplies, etc.
For a task to be internal, click Work at the top, and in your To Do tab, click Add task.
From there, you should see the menu below and be able to add a task as normal. If you do not know how to add a task normally, check the last half of Creating a task. From there, the task will appear on your To do list, after setting the appropriate Due by filter. If the task has no subtasks, you can mark it as complete once done (or reassign or snooze).