Traditionally manufacturers have been stuck with the Single Level Cell (SLC) approach whereby capacities only increase when a single, large (and therefore hugely expensive) slab of flash memory is employed. By contrast, MLC technology means using multiple smaller (and cheaper) flash modules together to create much larger storage capacities. The approach had been tried in the past but until now inefficiencies meant the performance drop-offs were so great that it wasn't a viable option.
If you still aren't getting it, try this: Much larger and cheaper SSDs are on the horizon with no performance sacrifices whatsoever