I had to research this to keep my wife from freaking out.
The yolk detaches from the ovary, and sometimes it can detach a bit soon, and a bit of blood or tissue comes along with the yolk. Like a loose tooth, sorta. The "old wives tale" was that meat spots came from frightened chickens. Maybe, if the farm dog was chasing them. It seems that it can happen from activity when the yolk is near the point of detaching.
Commercial white eggs NEVER have meat spots for various reasons:
-- Research (sorry, I don't have the links) suggest that brown eggs are more likely to have meat/blood spots
-- Commercial eggs are candled to identify them, and those are sold in different ways. White eggs are easier to candle with automatic machinery.
-- Battery hens are confined, so they don't have a chance to jump off the roost at just the wrong time of day.
-- As the hen matures, they are less likely to occur
-- As the egg ages, the blood is bleached out by oxygen, so it doesn't show.
They're fine to eat. If it bugs you, pick it out, or scramble the egg.