Geopolitics is the study of how a country's geography (location, terrain, land size, climate, soil and raw materials) affect its foreign, economic, military policy and strategy. The word geopolitics comes from the words "geography" and "politics".
Examples
Russia is a large country. Its land is mostly flat. This makes defending its borders (where no mountains or natural obstructions exist) a difficult task. It is expensive to build infrastructure in Russia. Its extreme climate makes poorly built roads break more often. The United Kingdom is a small island with a mild climate and access to warm water ports. It is easy to defend because any foreign country has to cross a body of water to attack. In addition, since it is small and warm the cost of construction is cheaper than in Russia.