What is "Arrival-time Alignment" and why is it important in home theater?
Arrival-time alignment (often incorrectly referred to as time delay or time alignment) is the process of ensuring that the direct sound from each speaker arrives at the listening point at the exact same moment.  This is achieved through speaker placement and signal delay.
 
Why is arrival-time alignment important?
Selective listening is inherent to human hearing, allowing us to "block out" sounds that are not perceived as important or relevant given the current situation.  In home theater, our brains must constantly distinguish between direct and reverberant sound. If direct sounds from a speaker arrive at a different time than those from another speaker, your brain will confuse the later as a reverberant sound and discard them as unimportant.  Precise arrival-time alignment drastically improves clarity and detail, making home theater a true experience.