People usually take this to mean the loss by death of a loved one or a person you held in some regard. As I write, and as you read this, people will die that you and I don't know and we feel no sense of loss. Naturally, were we to meet a relative who lost someone dear to them as this paper was being written or read we would feel sympathy and no doubt show kindness. Nevertheless, bereavement is about personal loss; usually of a person, but perhaps also a family pet, may trigger a deep feeling of loss.
There is a well established pattern to bereavement. Denial, Anger, Depression, Acceptance.
People can feel loss with regard to many things; most obviously, family break up. This can induce feelings of loss perhaps more intense than the loss of a parent, particularly in countries that insist on sexually biased parenting orders. Often successful people can become quite depressed at the pinnacle of their business life/career. Retirementcan also cause extreme feelings of loss.
The process to acceptance seems to be mediated by the severity of the loss, the persons experience of loss and their ability to manage that loss effectively.
Counselling comes in for those few individuals who do not move on; they may become stuck on any phase. We have all come across people who can't mention a lost ones name, the husband who keeps his departed wife's' car on the drive and clothes in the wardrobe. These are the people who need the help of psychologists or councillors.
The process needn't be long; often after just a couple of sessions the person is on their way. They will probably want to come back at various points, this should be made available, or built in to the therapeutic agreement.