Every time a company insider — an executive, board member, or major shareholder — buys or sells shares of their own company's stock, they must file a Form 4 with the SEC, generally within 2 business days of the transaction. That fast filing deadline makes Form 4 data close to real-time, a meaningful contrast to the months-long lag of standard quarterly or annual filings, and the data is entirely free and publicly searchable on SEC EDGAR.
It's genuinely important not to over-interpret a single Form 4, though. Insiders sell shares for many routine personal reasons that have nothing to do with their view of the company's prospects — diversifying concentrated wealth, paying taxes on vested stock awards, or funding a major personal purchase, among others. A single sale in isolation carries limited signal value. A cluster of MULTIPLE insiders independently buying around the same time is generally considered a somewhat more meaningful pattern, since it's less likely to reflect pure coincidence or one person's unrelated personal circumstances.
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