Basically, the Marines were sent northwest from the Reservoir to try and link up with 8th Army who were were heading east to try and link up with the Marines. The Marines made it as far as Yudam-ni when the Chinese hit 'em too hard for them to proceed. The Toktong Pass was the MSR and the only route to or from Hagaru-ri where most of the 1st Marine Division was located. Fox Co., 7th Marines held the pass so they wouldn't be cut off by the Chinese.
I found some photos of the Toktong Pass immersion gallery at the National Museum of the Marine Corps, and from another account of the museum display:
Beyond the atrium, visitors face combat.
In Korea's Chosin Reservoir, 250 men of the Fox Company hunkered along the icy Toktong Pass supply route, spending five days defending it from Chinese soldiers, said retired Col. Joseph Alexander, a Marines historian from Asheville, N.C., who consulted for the museum and wrote some 800 captions for the exhibits.
Half the men of Fox Company were killed.
To fully explain that standoff, the museum would have to import piles of dead bodies, plunge the temperature to 20 below zero, invoke frostbite in its visitors and keep them awake for days in foxholes, overwhelmed by the stench of human waste and death.
I was going to go into more detail of the actual fight, but that quote summed it up pretty good, I think. There is no dearth of information on this battle, so you can hit the links or the google if you wish to learn more.