How does SEO work?

Search engines are designed to return the most relevant pages for a user's query. In order to do this, the search engine needs to be able to read the content on websites and then present a list of results in order of relevance. 

Website optimisation ensures that Google can not only read and position your website correctly in search results, but that the search engine also recognises the semantics behind your webpage. 

As an example, it should be easy for visitors to recognise the different meaning between the word "apple" on a phone-repair website and a recipe website. By correctly optimising your website, you can ensure that Google can also tell the difference. 
Once Google knows what your website is about, it uses over 200 individual factors to decide where you should be positioned in the search results compared to your competitors. These are constantly being updated (with Google making multiple changes a day).

Some things to consider when reviewing your website:

Is your website mobile friendly?
How quickly does your website load?
Do you have a variety of quality websites linking to you?
Have you seen any broken links on your website?
Is your content of higher quality than your competitors'?