What is meant by interrupts in the context of operating systems?
When the OS runs a process such as "init", it would have sent signals to other sub-systems/device managers etc, like the memory, I/O devices, etc. The interrupt is just a way of telling the processor or the OS that it is ready, or it has new input, etc. So sometime during init, the OS is waiting for sub-systems to be ready, and an interrupt from these sub-systems would indicate exactly that. So that is what the "event" relates to. 

In regard to hardware or software interrupts, hardware interrupts are those which come from the hardware. Eg. I/O devices like keyboard, etc. A hardware interrupt causes the processor to save its state and begin execution of interrupt handler. On the other hand, software interrupts are instructions in the instruction set, which causes the processor to change its privilege level from user-supervisor, also known as context-switch. These details will be covered later on in the text book.