"On It"

"On It" is a feature available on certain Dashboards that allows a user to specify which items they are currently working on that have been assigned to as a resource.

"Task Dashboard vs Work List Dashboard"

It might appear that the Task Dashboard is identical to the Work List Dashboard but the Work List is all items that are currently assigned to the user.  Therefore it might appear as though the Work List is identical to the Task Dashboard when only tasks have been assigned to the user.  It is Best Practice to either use separate Dashboards for your assigned item (Ex. Tasks, To-Dos, Issues) or use the Work List dashboard only to view your assigned items in order to avoid confusion.