What is it?
Multiboxing is playing several game characters at the same time. You run several WoW clients on the same computer and duplicate keyboard and mouse to all these clients.
Why people do it?
For fun and profit.
What is fun?
You become a team lead - it’s nice to feel like a boss.
You can do hard things alone - kill elites, finish group quests, run instances.
You can level quickly and joyfully (especially in Classic WoW where mobs are hard).
You can revenge your PvP foes.
What is profit?
You can gather resources x2, x5, or even x8 at the same time.
You can farm gold x2…x8 as well.
Is multiboxing legal?
Sure. See https://eu.battle.net/support/en/article/24258.
Does it need another account?
No. You can use your existing Blizzard account. However, you will need to add one more WoW subscription.
Is multiboxing difficult?
No. You need to bind 1 key, make 2 macros, assign identical keybindings for damage/mounting/drinking, and install any keyboard broadcaster.
Is multiboxing free?
Yes. At least you can try it for free. WoW is free until 20 lvl, as well as some simple broadcasters.
How to try?
See this topic:
https://us.forums.blizzard.com/en/wow/t/how-to-start-multiboxing-in-14-minutes/