`@Valid` triggers Bean Validation (Jakarta Validation) for a request body/parameter based on annotations like `@NotNull`, `@Size`. Validation runs on the server, before your controller method fully processes the object; errors map to 400 responses (often via exception handlers).
Expanding on the short answer — what usually matters in practice:
A tiny example (an explanation template):
// Example: discuss trade-offs for "what-does-`@valid`-do-in-spring-and-where-does-v"
function explain() {
// Start from the core idea:
// `@Valid` triggers Bean Validation (Jakarta Validation) for a request body/parameter based
}