This jutsu is essentially a massive version of the Shadow Clone Technique. Instead of a few clones, it creates a few hundred clones (or a number of equal grandeur). This is considered a forbidden jutsu, because it can potentially be dangerous to the person performing it, seeing as how all of the clones are given an equal amount of chakra directly from the user. Naruto learned the technique from the scroll of forbidden seals written by the First Hokage.
Few people are able to use this technique effectively, due to the large amount of chakra it consumes. However, this weakness is not an issue to Naruto, due to his massive chakra reserves. Having such reserves allows him to create hundreds of clones while maintaining a decent supply of chakra in each one. This technique is also used by Naruto to train quickly, since all the clones share their experiences with Naruto, but it causes extreme fatigue with repeated use. Some times in the anime or manga, Naruto does not appear to distinguish between the Multiple and regular variations of the technique, and often simply says "Shadow Clone Technique" regardless of how many clones he uses.