A breakpoint is a point in a Drupal website's design where the layout of the page changes to accommodate different screen sizes. Breakpoints are used to ensure that a website looks good on any device, from a small mobile phone to a large desktop monitor. Breakpoints are set in the CSS code of a Drupal website, and they are triggered when the browser window size reaches a certain width. When the browser window size reaches the breakpoint, the layout of the page changes to fit the new size.
For example, a website may have a breakpoint at 800px. When the browser window is less than 800px wide, the website will display a single-column layout. When the browser window is wider than 800px, the website will display a two-column layout. This ensures that the website looks good on any device, regardless of the size of the browser window.
Breakpoints are an important part of responsive design, which is the practice of designing websites that look good on any device. Responsive design is a key part of creating a successful website in Drupal, and breakpoints are an essential part of the process.