How does it help? I do know that it's the length of time the valve is open. What's the advantage of longer duration? Or shorter duration? What does it have to do with valve overlap?

There's 2 ways to get more stuff through a valve that's open part time, and you can't change the pressure behind it:

1. Increase the size of the opening, either make the valve larger or open it farther; or

2. Hold it open longer.

Additionally, the longer the time the valve is open, the more time air has to rush past it; and since air has inertia, it is slow to start moving at the time the valve opens, and tends to keep on moving once it gets going. So, you open it earlier to get the air started; and you hold it open longer to allow it to keep rushing in. This helps ALOT as RPMs increase. In effect, the intake duration determines, more than any one other parameter, the engine's optimum RPM operating range. Too much duration makes the engine very inefficient at low RPMs, but it comes alive at higher RPMs; and vice versa.