What is the fixture.detectChanges() method in Angular testing, and when should you call it?

import { ComponentFixture, TestBed } from '@angular/core/testing';
import { Component } from '@angular/core';

@Component({
  template: '<div id="box" [class.active]="isActive">Box</div>'
})
class ToggleComponent {
  isActive = false;
}

describe('ToggleComponent', () => {
  it('should render correct class bindings on detectChanges', () => {
    const fixture = TestBed.createComponent(ToggleComponent);
    const comp = fixture.componentInstance;

    fixture.detectChanges();
    let div = fixture.nativeElement.querySelector('#box');
    expect(div.classList.contains('active')).toBeFalse();

    comp.isActive = true;
    expect(div.classList.contains('active')).toBeFalse();

    fixture.detectChanges();
    expect(div.classList.contains('active')).toBeTrue();
  });
});