Pattern printed on the side of a 35mm film cassette, which is used by the camera to sense the required ISO/ASA film-speed setting.
The film speed is sensed by means of electrical contacts pressing against the canister, so dirt can cause miss sensing.
DX-coding is meant to simplify photography; but if no over-ride is provided, it can be a pain for advanced photographers who want to deviate from the film manufacturer's recommendations.