The original PlayStation memory card held 128 KB of storage.
Sony introduced the card as a removable way to save game progress, because the console itself did not provide user-accessible persistent storage. The card used flash memory and connected through one of the PlayStation's controller ports.
That 128 KB was divided into 15 save blocks, with each block represented by an icon in the PlayStation's save-data browser. Games varied in how many blocks they needed: a small save might use one block, while larger games could consume several.
A common mix-up is confusing the memory card's capacity with the console's 2 MB of main RAM, its 512 KB BIOS ROM, or the CD-ROM's roughly 660 MB capacity. Later PlayStation consoles used much larger memory cards and eventually hard drives, but 128 KB was the standard capacity of the original PlayStation card.