This is the core commercial engine of Google's business model and means that sensitive personal information you generate in one service (e.g., a health query in Search or a private email topic in Gmail) can influence the ads you see everywhere, including on unrelated third-party websites.
Google collects a wide range of personal data including location history, search queries, emails (for feature delivery), browsing activity on third-party sites via Google's ad network, voice and audio data, and device identifiers, combining these across all Google services to build detailed user profiles. This cross-service data combination powers Google's advertising business and means your activity on one Google product can influence what you see on entirely different products and third-party websites. You can review and delete your data, turn off ad personalization, and manage location history at myaccount.google.com/data-and-privacy.