Reflow (also known as layout) is a browser process that recalculates the positions and dimensions of all elements in the DOM tree. This is one of the most expensive browser operations and occurs when changes affect the layout of elements, such as modifying dimensions, positions, or adding/removing elements. Common triggers include: 1) Reading layout properties (offsetWidth, clientHeight), 2) Adding/removing DOM elements, 3) Resizing the window, 4) Changing font size, and 5) Computing certain CSS properties. Minimizing reflows is crucial for maintaining good rendering performance.