OnPage-Optimisation is one of the two pillars of Search Engine Optimization. You can differentiate between the OnPage- and OffPage-Optimisation (also known as OnSite- and OffSite-Optimisation).
OnPage-Optimisation deals with all internal adjustments on your website. It describes optimisation strategies that can be carried out by yourself and which can not be influenced from the outside or by third parties.
The optimisation methods in OnPage-Optimisation are content-, structural- and technology-related. When it comes to the contextual and structural OnPage-Optimisation, you need to create unique, high-quality and subject-related content (also called “unique-content”) and then optimise this content’s structure, such as the formatting, headlines and more.
On the technical side, OnPage-Optimisation deals with optimising the source code of your website. This includes using search engine friendly and valid markup, the optimisation of specific meta-elements (for example the meta description), the alt-attributes of pictures, as well as the page titles (Title-element). Another part is the optimisation of the internal link structure to ensure that the Google-Bot can better crawl and index your site.
You will usually first work on the OnPage-Optimisation of a website before you proceed to the OffPage-Optimisation.