"The Holy Grail" is a weird phrase from the old stories of King Arthur, written in the 12-14th centuries.
It was a Christian relic which some of King Arthur's knights searched for. They went on a long journey, or quest, to find it. So today we use this phrase to mean something that is hard to find but is very precious.
Some stage plays lose all the money invested in them. Most will only survive, but not make a profit. Very few will make huge profits like the Phantom. So, the huge profit is the Holy Grail.
I don't know why they used "speak to". It's not really clear, and I wouldn't ever copy it. But think of another word that means almost the same thing: address.
- the commander addressed his troops.
"Address" can also mean "face up to" or "recognise", as in "we need to address the problem.
If you think of your Holy Grail as being a "problem", something that people want but can't have, then you can see the meaning of the phrase. This huge profit creates a conversation around the problem of profitability.
But now I would just forget about it. You don't need to ever use it.
Edit: forgot to say that the Holy Grail, although it was added to the King Arthur stories quite late, and the stories are all pure fiction anyway, is the object of Indiana Jones' quest in the film "The Last Crusade."