SEO is short for Search engine optimization.
SEO is important strategy to look into as it is a way that you websites position can be improved within the search engine results.
It is a way to construct your website so it can be found by more people when they are searching for terms related to what your website is providing. In turn potentially helping to increase traffic to your website and convert these into paying customers.
Another important article to read could be to find out why is SEO important for my website.