Single deployable means you ship one artifact as one unit (one version to build, test, and deploy). It simplifies releases and rollbacks and avoids cross-service version mismatches. The trade-off is a bigger blast radius when something goes wrong.
Expanding on the short answer — what usually matters in practice:
A tiny example (an explanation template):
// Example: discuss trade-offs for "what-does-“single-deployable”-mean-and-why-is-it"
function explain() {
// Start from the core idea:
// Single deployable means you ship one artifact as one unit (one version to build, test, and
}