Original: A chosen-plaintext attack (CPA) is an attack model for cryptanalysis which presumes that the attacker has the capability to choose arbitrary plaintexts to be encrypted and obtain the corresponding ciphertexts.
Simplified: A chosen-plaintext attack (CPA) is an attack model for cryptanalysis which assume that the attacker has the capability to choose random plaintexts to be encrypted and obtain the corresponding ciphertexts.