Word replacements is a way to change spoken or typed words into other words.
You can select to make the word replacement before or after TTS.
This can be used in conjunction with the Azure dictionary feature... if you had someones name spelled wrong in the dictionary so it is pronounced correctly you can then use word replacements to make the text appear correctly (if using word replacements before TTS disabled)
In the future there will be an option to make specific words be before or after TTS rather than all of them.