In more detail
The header string names the browser, its version and the operating system, for example Mozilla/5.0 (Windows NT 10.0; Win64; x64) ... Chrome/130.0. Much of it is historical baggage kept for compatibility. Sites use it to adapt content and to recognise crawlers such as Googlebot. Chromium-based browsers now send a reduced string and offer details on request through User-Agent Client Hints headers such as Sec-CH-UA.
Why it matters
It is easy to fake and, with browsers reducing the detail they send, is an unreliable way to identify devices. Code that sniffs the user agent breaks when browsers change their strings, so detecting the feature you need is more robust. Blocking or allowing bots by user agent alone is trivially bypassed; a real Googlebot is verified with a reverse DNS lookup followed by a forward lookup.
Example
Chrome 130 on Windows sends:
Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/130.0.0.0 Safari/537.36
Mozilla/5.0, AppleWebKit and Safari are compatibility tokens, the version is reduced to 130.0.0.0, and Windows NT 10.0 is also reported on Windows 11. Decode any string with the User-Agent Parser.