In a sense, all MLM programs are multi-tier. However, in online MLM, this term usually means you get paid for every sale the people in your downline make, X levels down. X being the number of tiers in the program.

For instance, in a 1 tier program, you would get paid for each sale that you make directly. In a 2 tier program, you’d get paid for your own sales as well as for the sales of the people you refer. In a 3 tier program, you’d get paid for your own sales, the sales of people you refer, as well as the sales the people they refer.