SEO is Search Engine Optimization. A web developer used it to optimize their website, which included web design, themes suitability, images, site engagement like social share, description, related keywords in addition to relevant content as well webpage loading speed. The end result is attracting Search Engines bringing in the online searches to your website to get a solution from their searching.
There are 2 types of SEO,
1.On Page SEO (Optimize directly on-site)
2.Off Page SEO (Optimize indirectly off-site)