Thread

Thread in computer science is the execution of running multiple tasks or programs at the same time. Each unit capable of executing code is called a thread.

The main thread is the one used by the browser to handle user events, render and paint the display, and to run the majority of the code that comprises a typical web page or app. Because these things are all happening in one thread, a slow website or app script slows down the entire browser; worse, if a site or app script enters an infinite loop, the entire browser will hang. This results in a frustrating, sluggish (or worse) user experience.