“Sheol has enlarged her throat and opened her mouth.” This is the figure of speech personification. Sheol is pictured as a woman who cannot get enough to eat, and many people are dying and thus becoming “food for Sheol.”
“beyond measure.” This continues the figure of speech personification. So many people are dying that Sheol has opened her mouth wider than can be measured so that she can eat all the dead people.
and into her will descend.” That is, into Sheol will descend the evil people of Jerusalem, which is also a “her” in this text.
“dignitaries.” The Hebrew is “splendor,” but in this context, it refers to the dignitaries, nobles, or leaders in Jerusalem.