Then What's Sevii Islands? Is it required?
It's required if you want to open up trade with Ruby, Sapphire, and Emerald after the E4 as well as catch Johto Pokemon and go through several side quests. Otherwise, you can skip it.
The Sevii Islands are a set of islands (three accessible before the E4, four more after, and two more that can only be accessed through Nintendo events -- so seven without events and nine with) that you can reach as soon as you defeat Blaine of Cinnabar Island. Just outside the Gym, Bill will spot you and invite you to come with him to the Sevii Islands to visit Celio, a friend of his. If you say yes, he'll take you to One Island, where you can start a side quest that takes you through the first three islands to find a little girl and deliver a meteorite. Then, after you defeat the E4, you can go back to the islands (via Vermilion Harbor, the same place where you used to be able to board the SS Anne), talk to Celio, and go through
another side quest to defeat Team Rocket and get a pair of plates Celio needs to complete a machine that lets you trade with Hoenn.
If you say no, then Bill just sits in the Cinnabar Pokemon Center, and you can go on to get the last badge and defeat the E4. You can still get to the islands if you go talk to him, but it doesn't change the course of the rest of the game if you don't.
