An accessible explanation of what 'Turing complete' means, tracing the concept back to Alan Turing's 1936 paper 'On Computable Numbers'. Covers the basics of computational theory, how a Turing machine works (tape + finite state machine), and the Church-Turing thesis. Explains practical implications: any Turing-complete system can compute what any other can (given time and memory), why unusual things like Conway's Game of Life or the x86 MOV instruction are Turing complete, and the security risks of unintended Turing completeness — illustrated by NSO's zero-click iMessage exploit that constructed a virtual machine inside a JBIG2 image decompressor.
