Angular map. What is it?

I am learning Angular 6 and I am puzzled with constructions like a:

this.contentArray.map((v: string, i: number) => `Content line ${i + 1}`)

or like a:

return this.aService.getItems()
    .pipe(map(response => response.data));

I am run through the couples of book like a “The_Complete_Book_on_Angular_6” or “Pro Angular 6” (Adam Freeman) but there are no simple explanations there. Google kept silence about that too. Can someone give the right and good tutorial or may be book (for amateurs) about array.map, array.filter and about .pipe(map(…))?