It's where Jack and Ennis meet, the "summer range" (4) for a herd of sheep that they spend several months in 1963 tending. It's basically just the two of them all summer, and the love that dare not speak its name comes into full bloom under Brokeback's lofty peaks.
In that sense, Brokeback is the most obvious symbol is of their love together: an idealized space full of campfire food, beautiful landscapes and steamy nights in the tent. Both men use "Brokeback" as a shorthand to describe the intensity of their feelings, and Ennis even says "Old Brokeback got us good and it sure ain't over" (67). You could just as easily sub in "love" for "Brokeback" and the sentence would make perfect sense.
And it's a telling phrase there: "got us." They can't stop—they're too much in love—and yet what happened in Brokeback was a once-in-a-lifetime thing. They're stuck in that memory because they can't have a life together and they can't express themselves publicly. All they can do is sneak off on the occasional fishing trip (that involves things other than fishing).
To quote Jack, "We could a had a good life together, a f***in real good life. You wouldn't do it, Ennis, so what we got now is Brokeback Mountain." (120) So Brokeback doesn't only represent their love, but the way they can't resurrect it because it's something that's in the rearview mirror permanently. They will only ever have that memory. It's a tough break—one might even say a backbreaking break (see what we did there?)—and one suspects that that's the whole point.