Your user agent (or useragent / user-agent as it is sometimes written) is the browser you are viewing this site in. Back in the day, to get online you had to jump through hoops and specify a lot of information to connect to a website. Browsers were invented to do this work for you. The website would need to know who you were and what you were looking for so browsers' user agents would send the user agent information with each request.
Systems can use this information to provide different views back to the browser. For example, by knowing that the user is using a mobile device we can show them smaller images and remove areas of websites that would be too wide for the device to show - this is called adaptive design, and it's something that Google do a lot with the search engine results.
There are ways to fake and alter your user agent, and if ever you do this set the browser and system to an old one and view Google's search results... it will look just as dated as the user agent is. This is because the user agent tells the server which browser and system you are using, and the server has been set up to respond differently for each one.
Your useragent is:
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We don't save, store or ask for any personal information. Your visit is logged but no identifying information is saved, only your visit. Cookies may be used as a marker to let us know if you've visited before. And thats about it.