Is there the sexism ... What is its causes?
Simply, it is treating another person or group differently purely because of their gender. Usually it is females discriminated against and being less favourable trated thaan men. (lower pay, poor education, forced marriage etc.)
Sexism like all other -isms, is the result of insecurity and fear.
Sexism is the unfounded and irrational conviction that one gender is inherently inferior to the other.