Indulgences

I am staying in Edinburgh, at a religious institution, and I pass a shrine to Our Lady on the way from the car park.

I see that I can get a 40 day Indulgence if I say a Hail Mary when passing the shrine.

That seeems reasonable. But I have a few questions. What is an Indulgence? What is the precise form of "Hail Mary"? Answer me these questions and I might be on to a winner!