A monolithic architecture is a single deployable application where modules run in one process and usually share one database. The system is built, tested and released as a unit, which is simple early on but can become tightly coupled as it grows.
Expanding on the short answer — what usually matters in practice:
A tiny example (an explanation template):
// Example: discuss trade-offs for "what-is-a-monolithic-architecture?"
function explain() {
// Start from the core idea:
// A monolith is a single deployable application where modules share one process and usually
}