SEO is Search Engine Optimisation. It refers to the way you structure your website to give it the best chance of appearing in 'organic' search results, i.e. those that appear below paid ads.
In general terms, it's important for your website to be structured in a way that makes it easy for search engines - like Google - to browse through your content and show your website to people asking questions related to your products, services or industry.
SEO is important in your website performing well in Search Results, however it's easy to get lost down that rabbit hole and forget about all other marketing that is just as important. Depending on your business & industry this could be print marketing, local publications, radio, networking, events, video, blogging or social media.
Because we specialise in helping small businesses with their websites, and we know you have limited time & resources, we always advise a bigger picture approach to marketing, using that to guide what specific types of advertising you should throw your energy at.
For more, check out this blog article: UpBound Blog - 3 Must-Have Features for A Website That Works.