The horizontal axis of the graph represents time, from 65,000 BCE (Before Current Era) to the present. Periods of 5,000 years are separated by vertical lines.
The vertical axis shows the total size of the world population, which has grown spectacularly over the period. Between 50,000 BCE and the present day, the number of human beings has risen from around 600,000 to 6.5 billion, a ten thousand-fold increase.
If that helps :)