Very basic but important questions.
Reference answer: https://en.wikipedia.org/wiki/Interrupt
Basically an interrupt is a signal to notify kernel(processor) that something needs to be handled as soon as possible. It's called interrupt because kernel(processor) may suspend what it is running to handle the interrupt event(by calling ISR) and then resume its execution.
There are two types of interrupts: hardware interrupt(which caused by external device, like keyboard, mouse, disk, etc) and software interrupt(which caused by program, like system call, divide-by-zero)