It predicts what you are going to type as you type. It has a tendency to add words to the end of a message when you hit send.

Man, gboard does that on android so much that I wound up installing and using heliboard. It kinda sucks but at least it doesn't "fix" your message after you type everything

Whatever it is, it's bad on GBoard, too, if you possess better than a 6th grade vocabulary.