If you have a website, particularly and e-commerce one, then you will probably find sites that offer external reviews of your products or services as well as how good the website and customer service is. These reviews will be left by people that have had experiences with your site and feel that they want to let other people know what you are like to deal with regardless of if it is good or bad.
Reviews can have a massive impact on your sales and return custom and plays an important role in marketing. When I have spoken to managers / owners of other companies, often they say that they do not really pay much attention to reviews left on other sites, but anyone who is serious about growing their business should. If these people are leaving negative comments, other potential customers may read these and decide to go elsewhere. You need to tackle the issues by firstly finding out what the complain was and then to come up with a plan to compact it to ensure it does not happen again. If you can do this and repl to the bad reviews, apologising and perhaps explaining that the situation is now rectified, you may find that they decide to give you another try.