When it comes to Search Engine Optimization (SEO) there are two main disciplines: OnPage Optimization and OffPage Optimization.
While OffPage basically describes link building and social media activities, OnPage goes along with all criteria that can be improved on the website itself such as crawlability (domain management, redirects, page speed, HTML, meta tags …), information architecture (keyword analysis, wording, landing pages, navigation, internal/external linking, URLs, …) and unique content.
If you know all the OnPage criteria that is relevant for a good ranking (check out the “OnPage Optimization Walkthrough” from searchenginejournal.com and the “Ultimate Guide to OnPage Optimization” from webdesignerledger.com to learn more) you can either optimize your website manually or you let the work be done by a SEO tool that indexes your website and tells you where exactly you need to fix and change something.