tsconfig.json configures the TypeScript compiler. strictNullChecks treats null/undefined as distinct types and prevents using them where a value is required.
Expanding on the short answer — what usually matters in practice:
A tiny example (an explanation template):
// Example: discuss trade-offs for "what-is-tsconfig.json-and-what-does-strictnullch"
function explain() {
// Start from the core idea:
// tsconfig.json configures the TypeScript compiler. strictNullChecks treats null/undefined a
}