A canister is a container which is roughly cylindrical in shape.
On March 3, 1986, Marty McFly found a canister in Doc's secret lab and used it to send a letter to Doc Brown. The letter let Doc know that Doc's wife Clara had sent Marty a letter from 1893, via several generations of Western Union, letting him know that Doc had disappeared into the future.
When Doc discovered the letter in his secret lab in 2035, he was confused since he wasn't in trouble. However, shortly afterward he lost his memory by an electric jolt shot by Officer Griff Tannen. Amnesiac and confused, Doc found Marty's letter, and went to the location and time specified therein.
Once he regained his memory, Doc gave Marty a bag of canisters, telling him to leave a note in a canister in his secret lab if he is ever in trouble.
On April 14, 1986, Marcus Irving broke into Doc's secret lab and left a note in a canister. Marty found the note, and came face to face with Irving. 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.
On April 15, 1986, Marty used the canister system to send Doc another letter. Irving had unnerved Marty, by telling him that the other versions of himself from different timelines still exist in alternate dimensions.
Marty wrote the letter to Doc to get his help, but when Doc arrived, Marcus used this to his advantage. 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 to allow his Yugo time machine to travel through time.
- Back to the Future (IDW Publishing)