Using separate yarn for each section?

HELP! I’m knitting a dog sweater and I have come to the “leg opening” part and in the notes it says "all three sections of leg openings are worked at the same time, using separate yarn for each section. What does this mean?

It means that each section will have it’s own strand of yarn. Use the other end of the ball, wind a little ball, or use a new skein if you have one.

THANK YOU, Ingrid!