The component is applied to the <my-app></my-app> tag in your index.html. If your index.html doesn’t have that tag Angular will fail at startup. You can control where you Angular application will be played.
This is special for the Angular component that is created using bootstrap(AppComponent)
The directive selector [myHighlight] will create aMyHighlight directive instance for all elements that have a myHighlight attribute like <xxx myHighlight> and where MyHighLight is listed in directives like
@Component({
selector: '...',
directives: [MyHighlight], ...
})
export class Xxx
Like the directive selector for other components (that are not the root component like AppComponent usually is), it works the same like the selector for the directive. The component has to be listed in the directives array. Then all tags that match the selector are upgraded to Angular components.
Selectors are like CSS selectors. They can be attribute selectors, tag selectors, class selectors, id selectors and combinations of these. Also :not(...) is supported.
What is not supported are selectors that need to match parent and child like with combinators like a b or a > b or a + b where b is a sibling, child, descandant, … of another component. A directive or component selector can always only refer to a single element.