Lightening-search.com is a website hosting a fake search engine. Most users would not land on it intentionally, and, instead, were forced to open it through unwanted redirects. Another possibility is the existence of a browser hijacker that has been installed on the user's computer. In that case, the homepage, new page tab, and the default search engine will be set to open lightening-search.com, and the presence of the browser hijacker will not allow these settings to be changed back.
Such browser hijacker applications aim to generate artificial traffic to their respective fake search engine and the generation of revenue through sponsored advertisements. Users should be careful when clicking on any of the advertisements displayed by these applications, as they may lead to fraudulent third-party websites.
As for why search engines such as Lightening-search.com are classified as fake, the answer is quite simple - they cannot produce relevant search results on their own. Instead, they rely on other legitimate search engines to do their job. In most instances, either search.yahoo.com or the Google Custom Search are used.