History is what’s happened to a task, over its lifetime. There are two views of history for a task.
When you have a list of Tasks in Workflow View just click on the pencil icon to open the details of that task, then select the history tab to display everything that has happened to that Task since its creation.
Searching by Date displays a list of every field edited at a particular date and time. This shows when things were changed, who changed them, and the new value at that time.
Searching by Field shows you a list, grouped by the individual fields, and what has happened to that Task in the same manner as above.