Some times a task comes in that is so important that everything else takes a step back. The hotlist helps you keep focus on those immediate tasks.
The hotlist is automatic. A task gets onto the hotlist if you give it Top Priority, or if it is due today or tomorrow. Remember you should only set a due date on a task if you have promised to someone else it will be done by that date, or if you need to do it to keep a plan on track. If you have a lot of bogus due dates in your task list, your hotlist will lose focus.
See also Start dates, due dates and repeats, and Setting priorities.