In general terms an overhead task is one which does not directly result in the modification or qualification of a required project deliverable. Under this definition most management activities, meetings, planning and monitoring are overheads, while specifying, designing, coding, documenting and testing are "payload" activities.
In xProcess there are 2 ways of specifying how a task will be scheduled: date-based or effort based.
Date-based tasks are generally more applicable to overheads. They run from a date to a date (the date may be derived from other dates such as project start and end or target dates) and have a specified time (or percentage of available time) to determine the effort required per day. If there are no resources available to carry out the task the start and end date are not changed - they will just consume less time in the schedule.
Effort-based tasks are the more normal type of task in xProcess. They have a specified size and required effort (see this link for the difference between size and effort). Their start and end dates are derived from the effort required and the availability of suitable resources (people who have matching role types and skills).
You don't have to use overhead tasks in your process - for example you could include in your estimates the time that will be required for these other activities. However it is useful to get a view from the time booked to overheads of the efficiency of your process and where improvements might be made.