It refers to the time a visitor has to wait until your page is completely loaded. On average, a page load for an e-commerce website takes 7 seconds meanwhile the ideal load time is around 3 seconds or less.
In 2010, Google announced that page speed would impact your website ranking. It has an impact on your audience’s user experience. A bad UX can cost you a loss of revenue if your target has to wait too long to get what they are looking for. They will just close your website. And above all, a slow page load is penalized by the search engines and has an impact on your ranking, both on mobile and desktop devices. Google says:
“A search result for a resource having a short load time relative to resources having longer load times can be promoted in a presentation order, and search results for the resources having longer load times can be demoted.”
Google compares your site to your competitors’ ones on a specific segment and attributes a bonus point whether you are the fastest or not. But be careful, that mark is not given unilaterally: it takes into account the country, device, or network.
So, there are a few factors you should focus on to improve your page speed.

How ti Improve it?
There is a few tools you can use to improve your page speed or simply to see how your website is working.
Page Speed Insight
This classic tool displays a page speed score between 0 and 100 and gives you all the keys and advices to improve your page load.
YUI Compressor
This tool will help you compress your code by minimizing JavaScript and CSS.
Webpage test
This Google tool offers a free speed test providing charts about page speed optimization.
YSlow
This plugin quickly shows you how your website is performing and gives you insights to improve your site speed.