This was first seen some weeks ago and I saw it earlier today in an email from Twitter. It shows you how to make a simple add-on yagi for your wi-fi router.
In my case I get sufficient range with the router "as is". You might find it useful if you need to increase the range in a certain direction. It is a simple idea, which I like.
See https://hackaday.com/2017/01/24/a-simple-yagi-antenna-for-your-wi-fi-router/ .