In general, SEO includes the actions for a website to appear in search engine results. It goes about on page optimization (titles, descriptions, keywords), off page (building backlinks, promotion in social networks etc.), technical things like website speed, robots file etc. and many more factors.
Another important part is content, it should be unique and relevant for your website niche.
If you never dealt with SEO, it's better to read the basics and learn it yourself or hire experts with experience in this sphere.