Here is a situation:
Suppose You were browsing through a huge Retail Site and You come across a product that catches your desire and now you just cannot live without. So you buy it and eagerly await its arrival.
When it arrives, You rip it open and see it - and find it's ... terrible. Disappointed, you go back to the same WebSite and leave a Negative Review to warn others and also this way you are informing the Company and venting your anger. Perhaps you are hoping that the Product be replaced et al.
You go along and share the same on your Social Media Platforms as well.
Now this is what can happen:
A few days later, you get a letter from a law firm representing the Product's manufacturer. It has decided your review constitutes defamation and libel. You can take down the review or you can show up in court.
The letter might even demand a small settlement fee for 'damages'.