So what is a MLM?
Start with a normal business. A company has some stuff to sell. Before the existence of media outlets, such as TV, Radio, Billboards, and so on, they pay a bunch of sales reps (consultant, advisor, distributor, member, associate...) to sell the product to people. That's a typical sales scenario. Tupperware was there long before MLM. You buy stuff from the company at wholesale prices, sell it locally at retail, and pocket the profits. There is no middlemen, so it is direct selling. As a traveling salesperson, you operate with low overhead, no storefront to worry about.
However, there's a problem for the company as that not all sales rep are equal. Some are good sellers, others are not. The company have to manage all the different reps, and it is complicated. Then someone came up with the idea... Why not let the reps manage other reps? Just reward the rep who recruited good reps by giving them a share of the profits. The more "lower" reps (the recruitee) sell, the more "higher" rep (the recruiter) make in bonus. So the recruiter has the incentive to recruit good people, which benefits both himself and the company. Thus is born "multi-level marketing", or MLM. The recruiter became known as the "upline", and the recruitee became the "downline".