Only you have the private key that can decipher them...
1) Click Generate: This will produce a public and private key
2) Share the public key with friends but keep your private key a secret
3) Your friends will then enter your public key along with their message, and send you back the encrypted message that the App produces
4) You will then enter your private key and your friend's encrypted message, and the App will decrypt it for you!
The RSA algorithm used in this app only produces primes between 200 and 500, so that the private key computation will be fast.
Therefor these encrypted messages can be cracked with powerful enough software.