The main idea of theories of international relations in general (as opposed to the main idea of each particular theory) is to explain the factors that impact international relations. There are many factors that could be used to explain and predict events in international relations. Theories of IR attempt to explain which of those possible factors actually do have an impact on what happens in international relations.
Different theories highlight different factors. For example, realism argues that the most important factor is the fact that the international order is anarchic; that there is no world government that can compel countries to obey laws. This means that international relations are governed by countries' desire to get power and security for themselves. Idealism, by contrast, argues that the international system is not anarchic and that there are many ways in which countries interact and cooperate so that they are doing something other than trying to get power.
In these ways, IR theories attempt to identify what factors are most important in determining what will happen in international relations.