turn a crank, grave apron

[kjc0123]

What is the meaning of "turn a crank", "grave apron" in the following sentences?

The man turned a crank under the grave apron. Very slowly, the casket began sinking. They watched it until it came to rest deep in the red soil.

[Clanad]

While andy hughes is quite close, a grave apron is a piece of material, often canvas and often colored green to look like grass, that covers the area of the grave surrounding the casket.  The casket sits on a mechanical device, as andy says, to lower it into the grave after the service.  I think the whole point of all this is to protect the sensitivities of the relatives of the deceased...