AES Crypto uses the symmetric-key encryption algorithm AES (Advanced Encryption Standard), a particular version of the Rijndael algorithm. AES has never been broken and is currently used by the US Government. AES Crypto uses 256-bits keys, which offers the protection level recommended by the NSA for TOP SECRET files. With AES Crypto, your data are secured.
Your secret data are encrypted and saved in your phone's internal memory. Someone with access to these files, including this app's creator, wouldn't be able to read them. No plain text file is stored on your phone.