Cutting ones nails:
Sparks of Kabala-Meaning behind nails:
The nails are rooted in a very high spiritual source. Prior to the sin of eating from the tree of knowledge Adam was completely covered by nails which protected him from evil. The sin then caused the nails to leave and only remain on the tips of the fingers, in which there also reside the impurities of the sin. Due to this the nails must be removed, and doing so brings peace to the world.
260/1; Michaber 260/1; Kol Bo 35; Rabbeinu Yonah Hayirah 309; Sefer Chassidim 58
 As the hand nails look repulsive when they are long.
Toe nails: However the toe nails which are not visible are not considered a Mitzvah to be cut Erev Shabbos. [Ketzos Hashulchan 73 footnote 4] However see Kaf Hachayim 250/17 which states that the Arizal would cut both his hand and toe nails in honor of Shabbos.
Admur ibid; Sefer Chassidim ibid
The severity of this Mitzvah: Elya Raba 260/4 records a Midrash which states “There was once a Rabbi who passed away and appeared to his students in a dream with a blemish on his forehead. He told them this is due to his negligence in speaking in middle of the blessing of Meiyn Sheva and Kaddish, and due to him not being careful to cut his nails Erev Shabbos”. This story is also recorded in Sefer Chassidim ibid
Shalah Hakadosh p. 138 in name of Tolas Yaakov