That's fotolibra’s new Quality Assessment Meter, a scale that shows you at a glance the enlargement potential of each image on the site. The more turquoise you can see, the better.
The rectangle simulates an A3 landscape sheet, or two facing pages of A4. This is very similar to the US paper size Tabloid.
In Preview mode we show the optimum print size for every image when printed in CMYK at 300dpi, standard commercial four-colour printing. This is the turquoise element of the graphic. If all you can see is turquoise, the image will be of high enough quality and resolution for virtually all purposes.
Do not confuse this with the sizes achievable by inkjet or dye sublimation printers, which will be able to produce very much larger images from the same file, for example poster prints.