Let's start with the filter operation. Filters is a function with a self-explanatory name; it filters in/out elements in the stream depending on whether the value satisfies a condition, as shown in the following example:

@Test
public void filterByNameReturnsCollectionFiltered() {
  List<String> names = Arrays.asList("Alex", "Paul", "Viktor",
         "Kobe", "Tom", "Andrea");
  List<String> filteredNames = Collections.emptyList();

  assertThat(filteredNames)
      .hasSize(2)
      .containsExactlyInAnyOrder("Alex", "Andrea");
}

One possibility for computing the list of filteredNames is the following:

List<String> filteredNames = names.stream()
      .filter(name -> name.startsWith("A"))
      .collect(Collectors.toList());

That one is the easiest. In a few words, filter filters the input and returns a value without all the elements filtered out. Using lambdas makes the code elegant and really easy to read.