SEO (search engine optimisation) is about getting your site to rank higher in Google for relevant queries. It involves things like: useful content, clear headings (H1, H2), fast loading, clean URLs (e.g. /services instead of /?id=7), and links from other sites.
At the design and build stage, a developer can: set page titles and meta descriptions, structure headings, optimize images (alt text, size), ensure responsiveness and HTTPS. That’s technical SEO and the base for further work.
Over time, fresh content (e.g. posts, updates) and external links matter too. SEO is a process — you usually see the first results after several weeks or months.