If you don't know, try it out.
What it does is double growth, production, and research for everyone. For example, at the start you only need 10 food to get another population point instead of 20, and only 5 shields to build a warrior instead of 10. This speeds up the game tremendously. However, it is a little off-balance since worker speed and unit movements are still the same. So even if you are industrious, your population grows alot faster than your workers can keep up with the terrain improvements.
With accelerated production, if you play at chieftain, then the AI will be building/growing at the normal human's pace (without accelerated production, or what your used to playing in previous patches), but the human would be building/growing at a pace higher than Deity.
With this I can see the AI launching spaceships around 500 A.D. on Deity!