Can someone help me in dummy terms distinguish what this definition means;
Monism is the metaphysical and theological view that all is one, that all reality is subsumed under the most fundamental category of being or existence.
Consider the difference in definitions between "monism" and "monotheism," for example:
http://www.answers.com/monism
http://www.answers.com/monotheism
They may seem the same on the surface, but they aren't. Once you can distinguish the differences, the definition of "monism" will be clearer to you.