Mask
noun
A temporary face is something you put over your face to cover it. It's a mask.
かめん
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
わぁ、びっくりした！何で仮面なんてかぶってるの？
Ah, you startled me! Why are you wearing that mask?
彼女は笑顔の仮面の下で、本当は泣いているのかもしれないよ。
Behind her smiling mask, she might actually be crying.
ハロウィンの日の夜、仮面ライダーのコスプレをしながら、チョコレートを一箱平らげたビエトを見ました。
On the Halloween night, I saw Viet wearing the Masked Rider costume and eating a whole box of chocolates.