Google Chrome's custom search engines are special search "engines", such that you can then enter something like techrepublic hit Tab, and then type a search string. What this does is search for the given string on the included site by using that specific URL's search engine.
For example, you enter techrepublic hit the Space bar; when the URL morphs into a box (Figure A), you type in the search string and hit Enter and the search results from techrepublic.com will appear.
The custom search I use the most is the one I've created to search 📝Charles Eisenstein's book,📝The Ascent of Humanity; the full text of which is published on his website. You can use it here.
References
- Google's Custom Search Engines, 📝Google
- How To Create Custom Search Engines In Google Chrome, makeuseof.com