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.
A special type of worker, called a service worker, can be created which can be left behind by a site—with the user's permission—to run even when the user isn't currently using that site. This is used to create sites capable of notifying the user when things happen while they're not actively engaged with a site. Such as notifying a user they have received new email even though they're not currently logged into their mail service.
Overall it can be observed these threads within our operating system are extremely helpful. They help minimize the context switching time, enables more efficient communication and allows further use of the multiprocessor architecture.