There is only so much oil in the ground — it is not being replaced, at least, not on a human time scale.
The term “peak oil” refers to the point in time when the rate of extraction of petroleum reach a peak, after which it enters a gradual but terminal decline.
Peak oil is a function of both geology — the physical amount of oil available — and economics. The costs to do with finding and developing new sources of oil are steadily rising because the easy-to-find sources were developed first. There comes a point when it is no longer economical to develop new sources. At that point in time peak oil has occurred.
Therefore, the term “peak oil” does not mean that we run out of oil; it means that we run out of affordable oil.
Imagine taking a magic carpet ride through northern Europe from east to west in Biblical. The area is circled on the map shown below — an area that Ugo Bardi rather unkindly refers to as a "vast regions of fog and swamps, inhabited by hairy Barbarians . . . the area we call today "Western Europe".