SEO stands for Search Engine Optimization and is the practice of increasing the number and quality of visitors to a website by improving rankings in the algorithmic search engine results.
There are two types of SEO. Organinc and non-organic SEO.
Organic SEO represents the ways you can use to improve your ranks onsite and offsite for free. This includes keywords, metadata, correct website structure and content, backlinks and more. It takes time to be effective but has no additional fees and will steadily increase your traffic.
Non-organic SEO is rank improvement by paying ads. In the short run will significantly increase your visitors but it has a high cost to maintain.