This is the 3851st most frequent Polish word.
ploski
"Płoski" translates to "flat" or "level" in English.
The word "płaski" is used as an adjective describing the flatness of the cat's tail in a simile.
Mój kot ma ogon płaski jak deska.
My cat has a tail flat as a board.
Here, "płaskie" describes the physical attribute (flatness) of the mirror.
To lustro na ścianie jest zupełnie płaskie.
That mirror on the wall is completely flat.
In this context, "płaskiej" is used as an adjective describing the moderate flatness of the plane in the spatial sense.