The Multiple Shadow Clone Technique is an advanced version of the regular Shadow Clone Technique. The difference between them is not in the essence of the technique itself, but in the number of clones created. Even a single shadow clone takes half of the user's entire chakra (due to the split), so the standard version is considered safe only for jonin and above. The Multiple version, meanwhile, creates dozens, hundreds at once, and in the case of Naruto Uzumaki, sometimes even a thousand copies, which requires a colossal expenditure of chakra. It is precisely because of this danger that the First Hokage declared the technique forbidden and sealed the information about it in the Scroll of Seals.
Naruto is one of the few exceptions: thanks to his enormous chakra reserves, including the chakra of the Nine-Tails, he can use this technique without threat to his life. Under ordinary conditions, the average shinobi rarely creates more than five shadow clones at a time, while Naruto freely operates with hundreds. Naruto himself often does not distinguish between the two techniques in his speech, calling either of them simply the "Shadow Clone Technique."
The main purpose of the many clones is attack: they surround the enemy from all sides and attack simultaneously, overwhelming with numbers. Aside from combat, Naruto used this technique for accelerated training. During his training of the Wind Style Rasengan, he created many clones in order to gain experience in parallel, and in a few days completed years of practice.
Likewise, it is precisely thanks to this technique that Naruto ultimately became an excellent master of taijutsu. Taking part in constant battles using shadow clones, he absorbed close-combat experience with incredible multipliers like a sponge.
There is also a special "Kakashi version": when Kakashi Hatake was exhausted and declared that he could not fight Gato's thugs, he nevertheless created dozens of his copies in order to scare the enemies with the mere sight of them and force them to retreat, although in reality they hardly amounted to anything.
