Degrees Plato (°P) is a gravity scale expressing dissolved wort extract as a percentage of sucrose by weight — rather than specific gravity's ratio-to-water approach, Plato asks directly "what percentage of this liquid's weight is dissolved sugar." It's the standard scale in German, Austrian, and much of Central European brewing literature and equipment.
The same underlying property, a different number line
A wort reading 1.050 on a specific-gravity hydrometer reads roughly 12.4°P on the Plato scale — not two different measurements, just two different ways of expressing the same dissolved-extract content. American homebrewers working mostly from specific-gravity recipes and calculators occasionally run into Plato figures when reading translated German brewing texts or European malt and yeast spec sheets, and being able to convert between the two mentally (roughly, gravity points divided by about 4 gives a rough Plato estimate) avoids real confusion.
Why it matters for the real-attenuation calculation specifically
Real attenuation — the more scientifically accurate alternative to apparent attenuation — is calculated using the Balling formula, which itself works in degrees Plato rather than specific gravity directly; the ABV calculator converts your OG and FG readings to Plato internally to run that calculation, even though the inputs and outputs you see are shown in specific gravity and percentage terms.