Pretty self-explanatory. I am using the built-in addon, Rigify to rig one of my characters. The rig isn't perfect so I need to go in and change some weights. I clicked on some of the bones to see what their name is, and I clicked on one that was called, "Jaw Master." For some reason, when I go to the vertex groups on my mesh for some reason I don't see a group of the same name. I saw one that was just called "Jaw" but when I moved the master jaw around it didn't do anything. So I am assuming that's the wrong group.
Rigify is a quite complicated rig, in which a lot of bones are "control" or "machinary" bones, so they don't deform the mesh (they don't have the "deform" option activated, so no associated vertex group is created when parenting).
All and the only bones who deform the mesh are stored into a generally hidden bone layer, and have the "DEF" suffix on their names.
So, probably the vertex group you're looking for is called "DEF-jaw".
Of course, after generating the rig, if you know what you are doing you can modify all of theese properties.