What it is
A witnessed cremation means the family is present at the point the cremation begins. Facilities that offer it usually have a small room with a window onto the charging area, or a screened space adjacent to it. Numbers are limited by the room, commonly to somewhere between two and ten people.
You are not present for the cremation itself, which takes hours. You are present for the beginning.
Who asks for it
Several traditions expect or prefer it. In Hindu practice a son or close male relative traditionally initiates the cremation, and many facilities will allow a family member to press the button. Some Buddhist practices include chanting at the start. Beyond religious practice, families where the death was sudden often want it simply to close the question of what happened.
This site's faith and cremation pages set out what specific traditions actually teach, sourced to their own documents.
What to ask before booking
Does the facility offer witnessing at all? How many people can be accommodated? Is there a separate fee? Can a family member start the process, or only observe? Is there space for a short reading or chant beforehand? And what is the scheduling window, given the 48-hour statutory wait?
What it is not
It is not a viewing. If the family wants to see and spend time with the person beforehand, that is a separate arrangement and it changes the service from a direct cremation into something else. Say which one you mean when you ask, because providers hear the two used interchangeably.