interface in c#

Question

 
"The CLR requires that interface methods be marked as virtual. If you do not explicitly mark the method as virtual in your source code, the compiler marks the method as virtual and sealed; this prevents a derived class from overriding the interface method. If you explicitly mark the method as virtual, the compiler marks the method as virtual (and leaves it unsealed); this allows a derived class to override the interface method. If an interface method is sealed, a derived class cannot override the method. However, a derived class can re-inherit the same interface and can provide its own implementation for the interface’s methods."

 

 

 

please i need explanation   of some point which is underline in above paragraph .

 

if we can not declare interface method as virtual then what is the meaning   of these line "If you explicitly mark the method as virtual, the compiler marks the method as virtual (and leaves it unsealed); this allows a derived class to override the interface method"