A "concession" in the oil industry is similar to a "claim" in mining -- it's a right, granted by a government, to explore for oil and extract it when found. The government gets a portion of the oil revenue based on the terms of the contract.
The way that oil extraction works, the license covers a specific area of land, ususlly government owned. That's what a "concession territory" is.
As the Wikipedia article about the event you refer to explains, the oil consortium called the Turkish Petroleum Company, later called the Iraq (not International) Petroleum Company, snapped up most of the concession territory in Iraq.
If a government feels that the terms are unfavorable, it can simply revoke the concession, and that's exactly what Iraq did.
IPC lost the right to develop oil under almost all of the territory that Iraq had previously granted it.
Iraq ran the risk of the companies convincing their own governments to go to war with it.
But this was the height of the Cold War, the American partners of IPC had already withdrawn to join ARAMCO, and Qasim was pivoting Iraq's allegiance towards the Soviet Union.