Multi-tabling is the simple act of playing more than one table of poker. Obviously that is impossible when playing live, however, when playing online you can play up to 24 at a time!
See how it is done with this great tutorial here:
Multi-tabling can increase your winrate by a factor of two or three if done right. After all, very few players currently play only one table online unless you are completely new to the game.
See how Nanonoko, a professional poker player, plays up to 24 tables at once:
Hopefully this blog will be able to give you some insights into how to multitable better than everyone else.
If your looking to start learning poker from scratch check out this great instructables article.
Win big at the tables!