What are "workers" in a web application?

I just have a little idea about it that the task is distributed among the "workers" and they help balancing the load of the task.
Since I'm a beginner, could you just tell me about workers, are they piece of software, library, hardware. Are they separate from an app? Are they add-on to a web app? Why are they exactly used for? How would you add a worker in your app?

Thanks in advance.

Since you posted in the WebSphere forum, is this question about some WS-specific feature?

If not, the concept of web workers comes to mind, but that's a client-side feature.