Marty's letter was an urgent message sent through time by Marty McFly in 1931 to Emmett Brown in 1985, to let him know that he was stranded in 1931 without a time machine, since Edna Strickland stole the temporal duplicate DeLorean.
Marty did not know that the DeLorean Edna stole was a temporal duplicate, as Doc never told him in that timeline. Thus, he had told Doc to build a second DeLorean.
December 18, 1931
It's me, Marty. You already know from our meeting in 1955 that your time machine will work. I can't explain now, but the me from 1986 is stuck in 1931, without any way to get home. So you need to modify a SECOND DeLorean — make it an exact duplicate of the one you're making now, and keep it well hidden.
Eventually, you'll know when to use it. Then come find me outside the Western Union Office in Hill Valley on December 19, 1931 at 9:35 a.m.
Hope to see you soon!
Your friend in time,