Heh, usually they mean "master copies." Like copying from Renaissance masters like Da Vinci and Michelangelo, but not necessarily other comics artists. Comics artists usually distort their figures somewhat, so you won't learn as much about what people look like by copying other comics artists. It is, rather, a good way of figuring out what you like about the work of your favorite comics artists and incorporating some of it into your own work.
But first you have to learn how to see.
I took a figure drawing class at a very good art school and for homework we always did master copies from artists like Da Vinci, Michelangelo, Villpu, and Rubens. I particularly loved the figure drawings by Paul Cadmus and John Singer Sargent. By the end of the semester, we had filled one and half sketchbooks with such copies.
Your work so far isn't bad at all - you've done a good job copying the look of each character. Perspective on a figure isn't so much an issue until you start doing whole-body shots. Right now your figures look flat because the anatomy isn't very convincing. Just takes practice, that's all.