In 3D gaming, polygons are everything. Much has changed since the early days of 3D gaming, but if you compare an early 3D game (like Doom) to a current game (like Crysis 3), the biggest difference you will see is the number of polygons on the screen.
In the early days of 3D gaming, increasing the number of polygons (called the “poly count”) was simply a matter of throwing more transistors on the GPU and ramping up the clock speed.
For awhile, this worked just fine, and we got games with higher and higher poly counts. But eventually, hardware designers hit a point of diminishing returns, and poly counts were unable to keep up with demand. So they started to get creative.