With this service you can record a message on the Bitcoin blockchain. This message can never be removed or modified by anyone. A possible use-case is to record a hash of a file, so you can prove that file existed at a certain time.
Giving an e-mail address is optional, but recommended because you will receive an email with a link to the transaction that contains your message so you can easily find it again later.
The price for this service is 0.0001 BTC.