It's a bit confusing. Once you sign into iCloud on both your iPhone and iPad, your phone number will show up on the iPad under "Send & Receive" in Messages settings. So if someone sends an iMessage to your phone number, it will show up on both the iPhone and iPad now instead of just the phone. You can't send regular (non-iMessage) texts from the iPad, but you can use your phone number on the iPad as your identity.