Search engine optimisation is the knowledge of a how a search engine works, and utilising it to your advantage.
Each search engine crawls a website taking into account thousands of different factors, and it's these factors which are used to position a website in search results.
Many of the factors look to see the quality of a website. For instance, if a website is well crafted and thought out.
By knowing what factors search engines take into account, it's possible to optimise a website for certain keyword/search phrases.