When filming something important, what visual aid, often shown as colored lines on your screen, helps you make sure your main subject is perfectly in focus?

The visual aid, often shown as colored lines on your screen, that helps ensure your main subject is perfectly in focus is called Focus Peaking. Focus peaking is a focusing assistant that overlays a distinct color, commonly red, green, or blue, onto the live view display of your camera's monitor or electronic viewfinder (EVF) to highlight areas of the image that are currently in sharp focus. When an area is in sharp focus, meaning it is rendered with maximum clarity and detail, it exhibits higher contrast at its edges compared to out-of-focus areas. Focus peaking detects these high-contrast edges and digitally outlines them with the chosen color. As you manually adjust the focus of your lens, these colored lines dynamically shift and appear on different parts of the frame, precisely indicating which elements are becoming sharper. This immediate visual feedback is particularly valuable when using manual focus lenses or when aiming for a shallow depth of field. A shallow depth of field refers to a narrow range of distance in an image where objects appear acceptably sharp, making it crucial to accurately identify the exact plane of focus. By observing where the colored lines appear, a filmmaker can accurately confirm that their main subject, and not unintended elements, is precisely and perfectly in focus.