Nature of something

[COUNTABLE/UNCOUNTABLE] a basic quality or feature of something
nature of: It’s the nature of plastic to melt under high temperatures.
They must understand the nature of our opposition to nuclear testing. (Macmillan dictionary)

I want to understand the meaning of "nature of something" e.g. Nature of disease, nature of life. , nature of our opposition.
Please explain to me the meaning in simple words of all these examples.
Thanks