It's the practice of building topical authority around meaning and relationships, rather than optimizing for isolated keywords. Search engines (like Google) now understand entities, context, and user intent—not just word matching.
Old SEO: "best running shoes" → optimize one page for that exact phrase.
Semantic SEO: "best running shoes" → build a content ecosystem covering cushioning, stability, pronation, race day vs. training, shoe materials, and expert comparisons.