Short Version
The number of pixels the card can render to the screen every second. Before pixel shader processing became the more limiting factor, this was the most accurate measure of performance (along with texel fill rate).
How To Calculate It
Pixel Fill Rate = (# of ROPs) x (Core Clock)
Does It Matter?
Only when comparing cards released prior to ~2003. Pixel Fill Rate scaled with performance until 2004, but it wasn't a direct indication of performance once DirectX 9 cards were released.
Long Version
Starting with the very first Voodoo 1, Pixel Fill Rate was 'king' when it came to describing video card 3D performance. This was before video cards did geometry processing, much less pixel shader processing. All a graphics card had to do back then was apply a couple of textures to a pixel and render it to the screen. And this is what pixel fill rate describes.
Starting with the GeForce 6 series, the number of Raster Operators got chopped down to make more room on the chip for pixel shader processors and pixel fill rate no longer scaled linearly with overall card performance.
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