Onpage SEO is the process of making your website better, particularly the content that you make available on it, how it is structured and categorised and how it all links together.
I also count technical SEO in this category, which includes making your website faster, following best practices and making sure things like SSL work properly.
There are several tools that can help you with this.
Best for Content Optimisation
SurferSEO is a remarkable piece of kit that compares your website’s content with your competitors and provides guidance about how to make your content correlate with content in higher positions in the SERPs.