What exactly is this Send as SMS option on my iphone 5s?

It says: "send as SMS when iMessage is unavailable. Carrier messaging rates may apply"

what the heck is this "send as SMS" thing? Should I keep it on or off? HALP

It means send a text message - so when you don't have internet access, and can't send an iMessage over the internet, it's giving you the option of sending the message as a text (or SMS) instead. You will be charged at the rate usually charged for such messages by the phone company you are signed up with.

SMS is the long form of saying a text message. What it means is if you can't send it as an iMessage the phone would then send it as a text message instead where of course your standard text message rates would apply.

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