the Berasit Hebrew Dictionary

כפרת

'keperet' is traditionally interpreted as “Mercy Seat”, which refers to the lid of the so-called “Ark of the Covenant”. According to Exodus 25, it was of pure gold and had a kerobim at each end.

'keperet' is formed by adding the et suffix to the root word, keper so it isn't really a word in its own right, but more likely a variant of keper. For further discussion see the article on affixes.