On page Search Engine Optimisation involves tweaking each separate page on your website in order to appear higher in search engine results pages and gain more traffic and increase it's relevance. Often changes are related to the coding of the website but also includes modifications to images and other aspects of the page.
Alterations to Googles algorithm recently meant that websites declined in both desktop and mobile rankings if they did not have a mobile friendly site. This was incorporated to encourage website owners to provide a better user experience for visitors to their websites and in acknowledgement to the drastic increase in mobile searches and people browsing the internet on their mobile devices. This is just one of the major changes that Google integrates over each year that web developers must adhere to in order to maintain positions on search results pages.