The left hand side of the diagram shows how much insolation is predicted from Milankovitch theory. The vertical axis measures off 600,000 years, and the horizontal axis measures how much solar energy should arrive at high northern latitudes.
The right hand side of the diagram indicates the volume of the Earth's ice sheets, as measured from the O16/O18 isotope ratio in ocean sediment. A long core sample was drilled out of the ocean floor, and the isotope ratio was measured at regular intervals along the core sample.
As you can see, the Earth's ice volume is currently quite low. The last ice age peaked 19,500 years ago, and abruptly melted away in just a few thousand years.
This diagram is from
What drives Glacial Cycles, W Broecker and G. Denton, Scientific American, January 1990, pp.43-50.