minimalmac:

Shortcuts let you create short combinations of letters that will produce a larger word or phrase when typed on your iPhone or iPad. Think of it as TextExpander lite. For example, you could type “omw” and have your device replace it with “On my way!” (Incidentally, that one is enabled by default. Try it.). Here’s how to create typing shortcuts on your iPhone, iPad or iPod touch.

Dave does a great job of explaining how to use this little known built-in feature in iOS.