A user agent is a string of data, including browser name, version, operating system and device type, that your browser sends to every website you connect with to help it customise the content display to your device.
Because the user-agent offers a range of information on every website visitor, marketers often use the user-agent data to cross verify their ad traffic against their targeting criterion. For example, checking whether the device type is mobile to pass the traffic as valid for a mobile ad campaign. However, the user-agent can be manipulated through a tactic called user-agent spoofing.
In user-agent spoofing, bad actors modify elements of the user agent string to obfuscate details of their traffic. For example, making high traffic volumes from a single device look like lots of individual advertising engagements from multiple devices. They could also make advertisers’ out-of-target traffic resemble their targeting criteria to pass the traffic as valid, like sending desktop traffic as iPhone traffic.
Source: TrafficGuard Glossary