Function to render Bootstrap 3 carousel markup from ProcessWire images object

bs3CarouselMarkup

// renders markup for a Bootstrap 3 Carousel from a ProcessWire images array
function bsCarouselMarkup($images) {
    $carouselId = $images->get('page').$images->get('field')->id;
    $carouselMarkup = "<div id='carousel-$carouselId' class='carousel slide' data-ride='carousel'>
    <ol class='carousel-indicators'>";
    $i = 0;
    foreach ($images as $image) :
        $class = ($i == 0) ? " class='active'" : "";
        $carouselMarkup .= "<li data-target='#carousel-$carouselId' data-slide-to='$i'$class ></li>";
        $i++;
    endforeach;
    $carouselMarkup .= "</ol>
    <div class='carousel-inner'>";
    $i = 0;
    foreach ($images as $image) :
        $class = ($i == 0) ? "active" : "";
        $carouselMarkup .= "<div class='item $class'>
          <img src='$image->url' alt='$image->description' width='$image->width' height ='$image->height'>
          <div class='carousel-caption'>
            <p>$image->description</p>
          </div>
        </div>";
        $i++;
    endforeach;
    $carouselMarkup .= "</div>";

    $carouselMarkup .= "<a class='left carousel-control' href='#carousel-$carouselId' data-slide='prev'>
    <span class='glyphicon glyphicon-chevron-left'></span>
    </a>
    <a class='right carousel-control' href='#carousel-$carouselId' data-slide='next'>
    <span class='glyphicon glyphicon-chevron-right'></span>
    </a>
    </div>";
    return $carouselMarkup;
}

That "get('field')->id;", which field is it?

thank you a lot.