Layouts are kind of templates files, but they do not refers to a particular view.
Let's call them headless layouts.

You can create any kind of tree structure inside the 'layouts' folder of your component, and then call them with an easy syntax.

Declaration

Have a look in the given example in your component:

layouts/form/fieldset.php

The layout recieve the datas trough $displayData array.

Instance the layout

echo JLayoutHelper::render('form.fieldset', array(
    'fieldset' => $fieldset
));

form.fieldset is the namespace wich will automatically load :
layouts/form/fieldset.php

learn more : JLayout