Search Engine Optimisation (generally always referred to as SEO) is the marketing discipline focused on growing visibility in organic (non-paid) search engine results. Basically, in plain English, it’s what helps you get to the top of search engines like Google, Bing, etc.
SEO is constantly changing as Google, and the other search engines, change their algorythms as to how they rank the website pages that they index. Long gone are the days where you stuff keywords all over you website and falsify the relevance of the web pages.
Nowadays it’s about making your web pages and website as focused and clear as possible on the subject for which you want to be found in the search engines for. For example, if you want to be found in the search engines for the term “Large Water Cans”, then it is important that your website and the specific web page, is clearly focused on “Large Water Cans”.
As well as the on-site optimisation, there is the off-site optimisation too. The main work here is building up a quality in-bound link campaign (only from quality, relevant websites).
The starting point for all SEO is keyword research.
The right keywords need to be relevant, ideally with a low level of competition (from other websites), whilst having a high level of monthly searches. In fact the most important part of that last sentence is the relevancy of the keyword. Sometimes it’s good to focus on long tail keywords (long tail keywords are search phrases that people enter into Google when searching for something. E.g. a long tail keyword for the large water cans would be “green large water cans in Hampshire”) as well, as the users using these longer phrases are often a lot further down their search journey, so are possibly easier to convert to customers.