When you're making stock or simmering meat in water,

[mercuryblues]

5. I found a few different answers

The foaming effect is caused by the water-soluble proteins and may be especially noticeable when cooking a product like corned beef or pastrami that is "enhanced" with a spiced or flavored solution.

"During cooking at high heat, some of that water comes back out of the meat and with the water is a small amount of sarcoplasmic protein. At high temperatures that protein denatures and assumes a foamy, frothy appearance," Sindelar explains.

https://madison.com/wsj/news/local/ask/curiosities/curiosities-when-boiling-meat-what-causes-foam-on-liquid/article_dcba823c-7296-11e0-bc10-001cc4c03286.html

Some people say that it has impurities that have collected in the marrow and other parts from being raised. I am inclined to believe this to a degree because when I use an organic chicken as opposed to a factory raised, I have less (if any) foam.

The one thing cooks agree on is that skimming it is up to you. The foam will make the broth cloudy, so if you are using it for a consommé it is best to skim. Others have indicated they can taste the difference between skimmed and unskimmed broth and those with sensitive stomach/digestive issues suggest skimming.