SEO stands for search engine optimisation. You engage in SEO when you want to get your website to show higher in the serps [ search engine results pages ], with the ultimate goal of increasing more traffic to your site as a result of higher positioning.
CTR is important especially if you have great page content and your listing titles and description don't fulfill expectations and bring visitors to that page. A low CTR is a signal to the search engines and based on the percentage can affect where this listing gets placed.
CTR stands for click through rate. In terms of the serps this means the percentage of people who click on a website listing and go through to the site itself.
LSI keywords stand for Latent Semantic Indexing. LSI keywords provide more context to a page. LSI is part of an alogoritm, it's designed to provide more relevancy to search intent by connecting keywords and creating more relevant options to the search term.