I think it is an American Football term (with same meaning as a Long Ball in Soccer). Obviously, it is difficult to throw the ball that far to the receivers, so the attempt to distribute the ball such a long distance down the field is speculative, with a low chance of success.
I guess that the significance is that the Long Pass was an early human interstellar slowboat carrying embryos and powered with its own fuel, while the faster ramships with the lethal ramscoop followed later with technology and materials. This was a precarious method of colonisation that frequently resulted in problems - Plateau, We Made It - so the mission was speculative and had a low chance of success.
Not sure if they would still play American Football or Soccer in 200 years time, but Larry Niven also named the first Quantum II hyperdrive starship The Long Shot which has the same meaning.
By the way, I thought Fleet of Worlds was an excellent book. I did write a review but it got eaten by the server crash last September.