Tracking changes is especially helpful when two or more people are working on a document. One person can suggest an edit or make a comment, and then the other person can decide what to do with it.
PC: Use the Review section of the Ribbon.
Mac: Click on Review, then move the Track Changes "button."
After people have marked up a document, a reader can see it without the notations by selecting No Markup or Original. But the edits are still there! If the next reader chooses All Markup or Simple Markup, then there they are. This can be very embarrassing when you're sending out a brief, a resume, or an important letter.
This article from Microsoft explains and includes a one-minute video walking you through the process.
When you have your document just the way you want it, save it as a PDF to send out. That way it will stay looking good and no one will accidentally change fonts or margins.