What is change detection in Angular in simple terms?

Change detection is how Angular keeps the screen in sync with your data. When something may have changed, Angular checks the bindings in your templates, such as {{ name }} or [disabled]="saving", compares them with the previous values, and updates the DOM only where the value is different.

Angular needs to know when to check. Historically, a library called zone.js patched browser APIs, so after a click, a timer or an HTTP response Angular ran a check automatically. Today, signals let Angular know exactly which views read a changed value, and zoneless apps rely on that instead of zone.js.

By default Angular checks the whole component tree from the top. With OnPush, a component is checked only when:

• an input reference changes,
• an event fires inside it,
• a signal it reads changes, or the async pipe emits,
• or you call markForCheck().

You rarely call change detection yourself. It runs for you after events and signal updates.

Think of it as Angular re-reading your bindings after something happens.