Take it this way, the CPU in the PCE is based on the 8-bit 65(C)02, with a 16-bit address space, so at any single instance, the maximum amount of memory it can "see" is only 64 kb.
However, through bank switching you can choose what it can see at any instance. The 64 kb space in the PCE is arranged into eight 8 kb "banks", such that you can switch in or switch out the content of each bank at any time you want. For example, a 1 Mbit (i.e. 128 kbyte) HuCard actually consists of sixteen such banks of data, so say, banks 1-4 of the cart contain data for level 1 of a game, and banks 5-8 contain that for level 2, so when you're in Level 1, banks 1-4 of the cart are switched into the main address space of the system so the CPU can access these data, and when you reach level 2, the CPU uses data from banks 5-8 instead, etc. (this example is of course oversimplified in an attempt to give a more straight-forward explanation).
For the memory buffer of the original CD-ROM system (64 kb total) or the Super CD-ROM system (256 kb), such memory can be switched into the main address space on the fly for the CPU to "see".
For the 2MB in the Arcade Card, unfortunately, things are a bit different(I don't know why, but there were some speculations that it's related to RAM speed or whatever). While the 2 MB are still arranged into a series of 8 kb banks, you cannot switch in these banks directly to the system's main address space for direct usage. Instead you need to copy the contents of these banks to the 256 kb SCD memory (or for graphic data, directly to VRAM) for use, like those RAM disks on computers, in which you need to "load" the files into the main RAM to use them. Fortunately copying from a AC bank to memory is more or less automated, thanks to those advanced bulk memory copy/move features in the CPU.
