SMS relay is part of Continuity and allows you to send and receive regular green bubble messages on both your iPad, and your Mac. As long as you're running iOS 8.1 on both your iPhone and iPad, you should be good to go to start using SMS relay. You'll just need to enable it first. Here's how!
In order to use SMS relay, your device must meet the following requirements —
Your iPhone must be running iOS 8.1 or later and have an active carrier plan that supports text messaging
Your iPad must be running iOS 8.1 or later
There are no model requirements for SMS relay though, which means any iOS device capable of running iOS 8.1 is capable of SMS relay.
How to send and receive normal carrier texts on your iPad
Make sure you're logged into iCloud on both your iPhone and your iPad — it must be the same iCloud account on both.
From your iPhone, launch the Settings app.
Tap on Messages.
Tap on Text Message Forwarding.
Turn On the option for forwarding to your iPad — remember you will only see it listed if it's signed into the same iCloud account.
The Messages app on your iPad should prompt you with a code. Enter this code on your iPhone when asked.
That's it! SMS relay will now be enabled on your iPad and you should start receiving regular green bubble texts alongside your iMessages. You can also send regular text messages from your iPad now, not just receive them. Just compose a message like you would any other time and your iPad can automatically identify whether or not it should send an iMessage or a text.