In your camera menu, you can enable a view called something like “highlight alerts”. Either by enabling it, or by pressing “DISP” or “INFO” until you see it.
This means that when reviewing, you get to see black and white blinking areas where the image is overexposed. Where it blinks is where it’s over-exposed.
You can also see this in software like Adobe Lightroom (By clicking on the arrow on the right of the histogram, in Lightroom’s case):
This means the dress is overexposed. All detail is lost on the red area. Bride not happy.
If you had left the camera to itself, it would have perhaps underexposed the dress, especially if it is large in the image:
Not bad, but bride still not happy, because a white dress should be pure, 100% white. Meaning it should edge against overexposing, but only just.
And that is where the blinkies come in. Enable the blinkies, and expose so that the dress is only blinking a little bit, and only just. In Lightroom, that looks like this:
And that is how you use the blinkies: expose so you see a tiny bit of blinking in the white areas – but only just.
The blinkies and the histogram represent a major contribution to photography – use them, and ace your exposures.