Marcus Irving's note was a letter that Marcus Irving left in a canister after breaking into Doc's secret lab.
Marty McFly, Doc's assistant, found the note, and came face to face with Irving on April 14, 1986. He revealed that, after his time travel project was terminated by the United States military, he used an inexpensive Yugo GX to create a partially working time machine.
Irving was searching for Doc in an effort to gain information that would allow him to finish his time machine. Marty was also looking for Doc in an attempt to find out why his memories did not reflect the current timeline.
Marty sent a message to Doc, who arrived shortly afterward. While Doc and Marty were talking, Marcus secretly removed the tires from the DeLorean time machine, and used Doc's flux capacitor to solve his problem with flux energy, which was the final step needed for time travel.
The note[]
IT'S TIME WE HAD A TALK
--A FRIEND IN TIME