Short Version
The clock speed of the onboard memory of the video card.
Long Version
The memory clock, along with the size of the memory bus, tells us the amount of memory bandwidth a graphics card has. The more memory bandwidth a card has, the better it can handle higher resolutions and high levels of AA and AF.
Memory comes in several different varieties, most of which are some form of DDR (double data rate). DDR memory can read from/write to memory twice every clock cycle. So if your DDR memory clock is 500MHz, the effective clock speed is 1000MHz. You'll notice, for cards that use DDR memory, the card pages show both the actual clock speed and the effective speed of the memory.
Like the core clock, the memory clock of most cards can be manually increased through the driver. Though highly overclockably memory is somewhat rare.
