The red areas are long term highest crime areas. The green areas are the safest areas. Yellow-orange areas have a medium amount of crime, long term. Orange-red have higher crime rates, long term.
The black dots are recent crimes, as differentiated from long term crime trends. You'll notice that when you mouse over to a black dot, it shows perhaps a couple of recent crimes and the nature of them. I haven't figured out how to see all the recent crimes as seen when you put your mouse pointer on the black dots.
Hope that helps. If you can, stay with the green areas that have the fewest black dots. That's your best bet for being in a safe neighborhood. Some folks who are honest, don't have much money, and sadly, they cannot afford to live away from higher crime areas.