It usually consists of ground beef, pork and veal mixed together. The ratio is usually 1/2 lean ground beef, 1/4 ground pork, 1/4 ground veal, but you may also see 1/3 beef, 1/3 pork, 1/3 veal.
Sometimes you’ll see recipes specifying other mixed meats, such as that including mutton, or sausage meat, but unless specified otherwise, assume it’s beef, pork and veal.
The texture of such a mix is much different than from just using all ground beef. The pork adds a sweetness, the veal adds a rich smoothness.
It is good for meatloaf, meat balls, and chilli. Such a mixture is sometimes found in hot-dog wieners as well.
Occasionally, the term “Mixed Meat” is used to mean “mixed grill”, as in many different types of meat being cooked separately but served together.
Wags point out that the mixture of beef, veal and pork manages to offend three major religions (Hinduism, Islam, Judaism) all at once.