via smashingmagazine.com => original post link
Web Workers are a powerful feature of modern web development and were introduced as part of the HTML5 specification in 2009. They were designed to provide a way to execute JavaScript code in the background, separate from the main execution thread of a web page, in order to improve performance and responsiveness.
The main thread is the single execution context that is responsible for rendering the UI, executing JavaScript code, and handling user interactions. In other words, JavaScript is “single-threaded”. This means that any time-consuming task, such as complex calculations or data processing that is executed, would block the main thread and cause the UI to freeze and become unresponsive.
This is where Web Workers come in.