Paste a user agent string and see which browser, operating system and device it describes - or press "Use my browser" to check yours, including the more accurate client hints.
It is a line of text your browser sends with every request, telling the website what software and device you are using. Sites use it to serve the right layout and to spot bots.
For historical reasons: early browsers copied Netscape's "Mozilla" name so websites would not block them, and the habit stuck. Chrome and Edge also mention "Safari" and "AppleWebKit" for the same reason.
Both send the same user agent. Browsers that support client hints can tell them apart - use "Use my browser" to see the exact Windows version for your own device.
Yes, it is easy to change, so never rely on it for security. It is fine for analytics, choosing a layout and rough bot detection.