The study of human population size, density, distribution, movement, and birth and death rates is demography. The graph shown gives us the estimated human population for several thousand years.
Notice that the graph shows a relatively stable number of individuals over thousands of years--until recently.
You can see the recovery of the population after the outbreak of the bubonic plague in the 1300s. It's estimated that 1/3 of the population of Europe died from the bubonic plague.
In 1804, the human population of Earth was estimated to be 1 billion people. By 1999 our population was recorded at 6 billion people. With a current growth rate of just over 80 million people per year, we are expected to reach a population of 8 billion by 2025 and 9 billion by 2050.