Why ARQ stands for Automatic Repeat reQuest. The receiving station checks each received block against a checksum or CRC, and if the block fails the check it asks the sender to transmit it again. This is an error correction scheme that relies on a return path, unlike forward error correction (FEC), which adds redundant bits so the receiver can repair errors without asking. Modes like packet radio (AX.25) and some HF data modes use ARQ to guarantee clean delivery.
Watch out The data compression choice invents an expansion of the letters; Q codes have nothing to do with it, and ARQ adds overhead for retries rather than shrinking the data. Encryption to obscure meaning is also generally prohibited on the amateur bands under Part 97.
ARQ = Automatic Repeat reQuest: "say again" in digital form.