Mylyn is a new paradigm in software development — Task focused development.
It greatly reduces information overload and makes multitasking easy. It synchronizes with a variety of Defect Tracking tools like Jira / Mantis / Bugzilla and integrates development context with the incoming tasks from the defect tracking tools. (See Eclipse Wiki on Mylyn Extensions for a list of external tools integrated with Mylyn.
You can find more at Mylyn Eclipse Page
Explaining Mylyn in words is very tricky. Seeing a screencast/video would be a better alternative.
Please Search for video of Mylyn in your favourite search engine and see them.