An inherent risk is a risk that comes "standard" with the project. For example, we know that software projects have always the risk of general technical difficulties. This risk is called an inherent risk, because it is "inherent" to the type of this project (it is known that software projects come with this particular risk).
Inherent risks are risks that exist in projects of the same type all the time (which means that you can't have a project of this type without the inherent risks that come with it).
Knowing the inherent risks of a certain project type will simplify the development of the risk management plan.
It's not easy to explain what an inherent risk, and I did my best. If you still don't understand then please let me know!