A user agent is a string it sends the website you're connecting to so it can know what rendering engine you have and which browser you use and so on. But, the dark side of user agents is that websites can use this to track you. So, if you don't want them to do so you can change it either manually or by using a browser extension. I recommend you to make your user agent look like chrome's, as this is the most popular browser and you will not be tracked as easily.