What do people mean when they say something is a good or bad source of protein?

Do they refer to what goes with the protein, like if it's meat with many saturated fats, it's a "bad source of protein"?

Or does it refer to the quality of the proteins themselves?

Does it have anything to do with completeness (if it contains all 5 essential amino acids)?

Although I can't fully answer your question, this website might help.

Biological Value: Protein Ratings

several variables.

one is whether its complete or not
another is more about whole food...what the protien to fat ratio (i.e. chicken is good, but fatty beef and pork is not)

sometimes its just in reference to a particular brand of protein powder, which then equates to: price per serving, taste, nutritional content (low carb, low fat, added aminos, etc)