A systemic constellation is a process by which:

 A Seeker: a person who wants some insight or information about a question or issue chooses…
Representatives: people to stand-in for family members, friends, colleagues, places or even concepts that are key players in the system they are inquiring into, with the help of a…
Facilitator: an experienced individual who guides the process

The seeker physically places the representatives in relation to one another (for example, side by side, or in front or behind one another, close to or further away). The seeker then sits down to observe.
The facilitator allows the representatives to feel into their roles with their bodies and then inquires into their physical and mental state. Eventually the representatives move into new positions relative to each other and more information becomes available.
Often the facilitator will suggest words that the representatives can say that may help reveal, reconcile and even resolve issues between them. Hearing representatives speak, and seeing them physically standing in new  orientations, can give the seeker useful and often healing insights.

At a high level, resolution comes in a constellation when, according to Hellinger, what has been pulled apart is joined again, or what has been inappropriately joined is separated. For example, a lost child is acknowledged – is seen and felt to belong to the family again, or a child who has taken on a parental role with a sibling is released from that inappropriate role in the social order.
This can be a healing experience for the system as a whole, as well as for the individual seeker.