A market force is a factor that has some ability to affect change in a market. Market forces determine the price and quantity of a good or service in a market. Market forces occur naturally in a free market economy and are controlled by government intervention.
An example of market force acting is when the price of crude oil increases when there are shortages in the supply. These shortages can happen for a number of reasons. The demand for crude oil could go up suddenly, or a disruption in the supply chain from a natural disaster like a hurricane (which happened in the United States when hurricane Katrina interrupted oil production in 2005. Additionally, an armed conflict (like a war) or a damaged pipeline can cause shortages. When these shortages occur, they become market forces. The demand outstrips supply which causes the prices to rise as the crude oil is less available and therefore consumers will be willing to pay more.
Assume that Country A gets all of its crude oil form Country B, a war breaks out in the eastern part of Country B where 30% of its oil production occurs. This war prevents oil from being produced from that region so Country B can only export the remaining 70% (of its production) of crude oil from the western part of their country.
Assume the war ends relatively soon, Country B will be able to export oil from the eastern part of their country again. This will result in an increase in the supply to the market and a return to the equilibrium where supply meets demand.