• SLC -> Single-Level Cell, i.e. 1 bit per cell
• MLC -> Multi-Level Cell, i.e. 2 bits per cell
• TLC -> Triple-Level Cell, i.e. 3 bits per cell
• QLC -> Quad-Level Cell, i.e. 4 bits per cell

The more bits per cell you store, the more dense and therefore cheaper your flash chips can be for a give capacity. The downside is that it is slower and less reliable since you have to be able to write and read exponentially more voltage states per cell, e.g. 2 states for SLC, 4 states for MLC, 8 states for TLC, etc.