Search Meter is a WordPress plugin that tracks what your readers are searching for on your blog. It can show you full details of recent searches or stats for the last day, week or month.
If you just want to install the latest released version of Search Meter, then go to your WordPress console and search for Search Meter.
This repo is where I hack on the code (you can fork and hack on it too). From time to time I make a new release to the wordpress.org plugin repository, and thence to the WP installs of thousands of happy users.