Organic traffic refers to the visitors that land on your website through unpaid search engine results. When a user types a query into Google and clicks on a regular search result (not an advertisement marked 'Ad' or 'Sponsored'), that visit is recorded as organic traffic.
Generating organic traffic is the primary goal of Search Engine Optimization (SEO). While it takes time and effort to build, organic traffic is highly sustainable and cost-effective over the long term, unlike Pay-Per-Click (PPC) traffic which stops immediately the moment you stop paying for ads.