Client Hints are an emerging web technology mainly being pushed by the Chromium dev team. Client Hints will let web browsers "hint" to web servers their various capabilities and properties (such as browser, version, operating system, screen size, bandwidth etc). This will let web servers serve appropriate assets depending on the device (eg. to save bandwidth, there's no need so send ultra-high res images to small mobile devices).
It's still early days for Client Hints, but we're watching the development closely and will be publishing articles covering it and how web developers can use them.
One of the things that Client Hints may start to do is replace user agents
As of June 2020, your web browser probably isn't even sending Client Hints! Currently the only browser that supports them is Chrome and you still have to enable them via a hidden developers setting.
In preparation for the dawn of Client Hints, we've already built a Client Hints Detection page to show you what's being sent by your browser. If you're curious, check it out.