Multi-Level-Cell (MLC) und Single-Level-Cell (SLC) are two different types of storage cells or generally speaking of NAND-Flash Storage.
SLC stores exactly one bit per cell. Costs more, however consumes less energy. Transfers data a little faster and all-in-all is more durable than MLC.
MLC stores more than one bit per cell. The main benefit is a higher storage density, since more than one bit per cell can be stored. So the double amount or even still larger amounts of information can be stored on the same chip surface as compared to single-level-storage. Especially with solid state memory this offers a significant price advantage in comparison to SLC, since the required chip surface is a very important cost factor in the manufacturing process.