Network throttling is an intentional slowing down of internet speed. In web performance, network throttling, or network condition emulation, it is used to emulate low bandwidth conditions experienced by likely a large segment of a site's target user base.
It’s important not to overlook network conditions users experience on mobile. The internet speeds for developers creating web applications in a corporate office building on a powerful computer are generally very fast. As a developer, tech writer, or designer, this is likely your experience. The network speeds of a mobile user accessing that web application, possibly while traveling or in a remote area with poor data plan covereage, will likely be very slow, if they are able to get online at all. Network throttling enables a developer to emulate an experience of a user. Most browser developer tools, such as the browser inspector, provide a function to emulate different network conditions. By emulating your user's experience via network throttling, you can more readily identify and fix load time issues.
In a browser's developer tools, under the network panel, there is generally a drop down list with network speed options including wifi, good 3G, 2G, etc., with a default value of "no throttling". You can even test 'offline'.