Short definition
Search intent is the underlying goal behind a query — what the searcher actually wants to do, learn, or buy when they type.
In depth
Search intent is grouped into informational ('what is local SEO'), navigational ('GoHighLevel login'), commercial ('best CRM for HVAC'), and transactional ('hire roofing contractor [city]'). Matching content type to intent is the single most important on-page SEO decision. A service-business homepage targeting an informational query will lose to articles; an article targeting transactional intent will lose to a service page.
Example
An agency rewrites their /pricing page hero from FAQs to a price grid after spotting that the query is transactional, not informational. Ranks jump from page 2 to top 3.