Ohhh, that's a good anime!
Sword Art Online takes place in the future, where the NerveGear is invented(it's like a headset that takes your mind into a virtual game so that you feel like you're actually in that place), and a new game called Sword Art Online is released. A few players were able to beta it before it came out, and among the group is Kirigaya Kazuto. The game is sold out immediately after it is displayed to public, and thousands of players log on for the first time. Kirigaya, who goes by Kirito in the game, is one of the few who have actually been able to explore the game, making him one of the only players knowing what to do.
When Kirito decides to log out for the day, he realizes he can't. The log out button is gone. There is a call to the main square, and when all of the players are gathered, their self-made characters transform into their real selves. The creator announces that no one can escape the game. A death in the game will contribute to their death in the real world, and if there are attempts to remove their Nerve Gear in the real world, it will fry their brain, also killing them. The only way to return alive is to fight their way up the 100th Floor and defeat the final boss.
It's a really good anime, although I dislike the relationship in it, but you really should check it out!
I watched it on the Crunchyroll app, but beware, it has ads. However, it's really easy to access and the layout is good, so I do enjoy using Crunchyroll. I also use it to watch anime on the computer, and I just get rid of the ads with Adblock.
Have fun! :D