What is SLOT-1 memory?

Probably referring to the Nintendo Ds and DS lite which had 2 slots. Slot 1 was the main slot for DS games while slot 2 could receive Gameboy Advance games and peripherals like a Guitar Hero adapter. Memory might refer to using 3rd party hardware which holds micro sd cards to store the pirate games to play on the ds.

Micro SD fits into 3rd party Cart. Cart fits into Nintendo DS slot 1.