Zero-sum refers to situations/transactions/environments where the resource in question is actually or just perceived to be fixed in size and therefore cannot be enlarged.  As a result, competition is more intense:  If I get 80% of the resource, you can only have 20%, or everything that I "win," you must "lose."  Humans tended to view economics exclusively in this manner until the Industrial--and "industrious"--Revolution began at the start of the 19th Century.  A good example is the concept of mercantilism--as in, the only "good" trade is that which generates a surplus of a precious commodity (throughout history, the focus here was on accumulating gold, a perceived fixed-sum resource because the world's supply grew irregularly).  Until the Industrial Revolution alerted humanity to the possibilities of escaping the limits of organic growth by creating new resources (i.e., the 19th century is considered the century of chemistry, resulting in all sorts of new chemicals and compounds, as well as substances and production processes made possible by them), Malthusian logic held (the notion posited by scientist and philosopher Thomas Malthus that wealth and demographic growth were inversely related--meaning, the more people a society accumulated, the poorer it became in aggregate, because there was only so much wealth to go around).  But with the Industrial Revolution, the causal relationship between population and economic wealth was broken: portions of humanity (primarily the West) got very rich and populous (exporting immigrants globally).  Now, as globalization spreads to those parts of the world previously denied deep economic connectivity, new Malthusian fears arise, creating suspicions of future zero-sum contests over resources.  But, as in the past, such fears will prove groundless:  when certain resources become "exhausted" in the sense that the cost of accessing them becomes too high (like oil), humanity will move on to new technologies that exploit resources in different and more efficient (and less pollutive) ways.  As a final note, when it comes to matters of threat, consider defense to be more zero-sum in perception (i.e., the more defense I have, the less you perceive yourself to have), while the interdependency of globalization shifts the matter from individual (or even collective) defense to that of shared security, which is truly nonzero-sum (i.e., the more security I build into my system, or into yours, the safer we both are).