To encrypt the messages you use a bookmarklet that takes you to the encipher.it website, there you will find a box where to enter text, the same box is used for encryption and decryption, when the recipient gets a coded message a one line phrase with a link link tells them where to decrypt it, the bad news is that they will need to know the password too, this should have been transmitted previously through a secure channel, attachments can not be encrypted.
Encipher.it can be used anywhere, i.e. Internet cafes and libraries, but anything you type in the keyboard of a computer that isn’t yours could easily be recorded, when you use a public computer you should not trust anything on it, if the website were to become too popular it might be easily blocked by an authoritarian regime ISP but that isn’t likely because copycats would then sprout around.
I would only be happy to use this browser based symmetric encryption system if the other part is very lazy about security or doesn’t want to learn about it and the security needs are low. Encrypting text, even with something as simple as ROT13 will leave the average computer users scratching their heads and it will fool email services that scan email messages to introduce contextual advertising on them, like Gmail, for serious security it is better using a public key encryption based system, i.e. PGP, what encipher.it has on its favour is convenience and easy of use.