Text Replacement is an iOS feature that automatically replaces a phrase with another, usually longer, phrase. An example would be to have xeml replaced by your email address. That way you could write only four letters and have it replaced by your email address, which would save you from writing a bunch of more letters. If you do it often it adds up and can save a lot of time.
Text replacement works throughout iOS. Among many other places you can use it in email, iMessage, Safari, Numbers, Pages and Keynote.
You can find iOS Text Replacement in Settings > General > Keyboard > Text Replacement.
It is currently not possible to put line breaks in replacement texts.