How do I determine the “key assumptions” of a startup

I understand the meaning and significance of testing the key assumptions,

In order for the idea to be viable, the key assumptions must hold true. These key assumptions need to be validated in initial user research (in my own words).

But I am somewhat at a loss in listing out what the key assumptions actually are. For me it is just the solution to a problem that annoys me.

Any tips or links to useful resources will be much appreciated.