It could be a few different things. I'm guessing you're talking about coughing up flem, as in during a chest cold, or cold? In that case it's either mucous/pus and maybe some bacteria and dead cells- if it's a viral infection, there usually won't be any pus, just a clear viscous sort of mucous. Either way, it indicates some sort of inflammatory response to a virus/bug/allergy etc. It's main function is to wash your respiratory tract out. That's why it's good to get it out - and why sometimes taking any old cough medicine isn't a good idea. Expectorants are best in this case. Did that answer your question?
