Html::checkboxList usage example

I try use Html::checkboxList from The Definitive Guide to Yii 2.0

<?= Html::checkboxList('roles', [16, 42], ArrayHelper::map($roleModels, 'id', 'name')) ?>

I don’t know what it is:

[16, 42]

Scroll down a bit and you will see checkboxlist info.

public static string checkboxList ( $name, $selection = null, $items = [], $options = [] )

So [16,42] are the currently selected items. If I had a list of checkboxitems and loaded the page, the items that have values of 16 & 42 would appear checked.

Thanks for answer.

I understand.