There are a huge array of marketing techniques and avenues that you can choose to do when trying to boost your business. Some are online and others are offline and depending on what you are trying to achieve you may try either one or both of these techniques. If you have a website then you may throw a huge chunk of your marketing towards it.
On page optimisation is essentially making changes on a page within your site or by adding new content to your site. It may be that you add a blog or that you optimisation the content on a page in include more keywords or build up internal links. Off page optimisation is marketing that you do offsite away from your website. The aim of this is to improve the traffic to your site and your rankings on the search engines. This can be achieved in a number of ways. Often people build up external links to their site from high quality authority domains, comments on other sites, have social media pages and interact with others or upload videos to sites such as YouTube. All of this can help build up your website without affecting the content on your site.
Ideally you want to do both to give your site the best chance of improving and gaining additional traffic.