Send Secure encodes your SMS before it leaves your phone, and only decodes it when it reaches your recipient's phone. Anyone else trying to intrude on your communication will only see meaningless characters. (e.g. %$fsd^#r3489y#$FSD)
Make sure your recipient also has "Send Secure" installed so they'll be able to decode your message.
Note: "Send Secure" encrypts and sends SMS using the normal SMS channels, so sending SMS is not free and is billed to you by your service provider at the normal cost