Check out example codes for "'mat-checkbox' is not a known element". It will help you in understanding the concepts better.

Code Example 1

import { MatCheckboxModule } from '@angular/material/checkbox';

TestBed.configureTestingModule({
   imports: [
        MatCheckboxModule
   ],
   declarations: [AddAlarmsFormComponent]
})

Learn ReactJs, React Native from akashmittal.com