This is when the production of crude oil peaks. There will still be lots left, but production is less as it gets progressively harder to get at. Historically this happens when about 50% of recoverable oil has been obtained. See Hubbert peak theory on Wikipedia for a primer and The Oil Drum and the The Association for the Study of Peak Oil for more details.
Authorities that I respect give the peak year between 2005-2010, though some oil companies and government organisations give a later date. The exact date is not too important, it is clear that we are already in a regime were the potential demand outstrips supply so that the price has to rise to bring them into balance, there seems little likelihood of increasing the supply significantly in the short term, so minor disruptions to supply (e.g. Prudhoe Bay) can cause a large rise in the markets (although for Prudhoe Bay it does not seem to have caused the price to rise much yet).
Peak natural gas production is likely to occur a few years later.
For five of the last six years grain production has not met demand and the stockpiles have reached a low of 57 days. To put the 57 days in historical perspective, the world price for wheat went up six-fold in 1973, the last time reserves were this low.
Of particular concern is China, where in 2003 grain production had been falling for 5 years China: Falling grain output raises concern
As glaciers melt and aquifers are expended we have more water available now that in the future. Better use of water (most water is used in very inefficient irrigation) and exploitation of remote rivers might stop this being too much of a problem.
The world's population is expected to peak at somewhere under nine billion in the middle of this century. This means that there are only two billion more people to add to the population.