Measure which of two shots holds more fine detail — overall and area by area — with focus peaking to show what is actually in focus, so picking the keeper from a burst does not rely on squinting.
How sharpness is measured
Two standard measures are used. The detail figure is the variance of the Laplacian — how much the picture changes from one pixel to the next, which drops quickly when a picture is soft. Edge strength (Tenengrad) averages the strength of edges. Both are taken after the two shots are placed on the same frame at the same size, so a 24-megapixel file does not win just for having more pixels than a phone copy.
The picture is also split into a grid and each area is measured separately. That matters when the focus point differs: one shot may be sharper on the face and the other on the background, and the panel says what share of the detailed areas each one leads.
Focus peaking
Camera makers borrowed focus peaking from video: edges that are in sharp focus are painted in a bright colour. Turn it on and the two pictures show where their focus actually landed — on the eyes, on the ear, on the wall behind. It is often a quicker answer than any number.
What the numbers cannot tell you
Noise and heavy sharpening also raise these figures, because grain and halos are fine detail too. The panel warns when one picture is noticeably noisier, and the peaking view helps tell real focus from grain. Motion blur direction is not measured, and a deliberately soft portrait is not a worse picture for measuring lower.