When a person makes a comment about a person or business that is harmful and false, it is considered to be defamatory. If someone, for example, goes to a restaurant and has a bad experience and writes a public review, it is perfectly legal. If they write a bad review but didn't have a bad experience, or exaggerate a bad experience to make it sound horrendous, that is defamation.
But what about someone who says something personal about you? If someone comments on Facebook that they always thought you were mean, it is their opinion and is not illegal. If they say that you cheat on your spouse and steal from your employer, as long as those things aren't true, it is illegal. Statements like that can split up families and ruin businesses.