analytics.js is the Universal Analytics JavaScript library, which allows websites to measure how users interact with their content. It sends hits (e.g., pageviews, events, transactions) to Google Analytics servers. This script helps website owners understand user behavior, traffic sources, and conversion paths. The script loads asynchronously and tracks various user interactions, such as page views, button clicks, form submissions, and e-commerce transactions. It uses cookies to store information about user sessions and to distinguish unique users, enabling the tracking of returning visitors and session-based metrics. Data collected is then processed and made available in Google Analytics reports, providing insights into website performance and user engagement.