Skin
The skin under your skin is still skin. The extra layer is there to make sure when you get little cuts they don't bleed all over the place.
ひふ
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
ギターを弾くので指先の皮膚が厚いんです。
The skin on my fingertips is thick because I play the guitar.