|This article is about an older version of DF.|
Speed is a numerical indicator of how fast something moves with lower numbers being faster. This is the value used in the Creature token for SPEED, not the same value as that which appears on the screen as Speed in Adventure mode. (In the adventure mode speed stat, higher numbers are faster.)
The way it works is that the game doesn't let most creatures act every single frame - an "average" creature acts only once every 10 frames. In order to achieve this, there is a delay on every action that a creature takes.
Basically, the hundreds digit and higher represent whole turns that are skipped just filling up the waiting counter until it reaches the point of their "speed". "Speed" is a bit of a misnomer, actually, as it's really more "turn delay".
This is why most creatures have 900 speed - they are delayed 9 turns before they get their 1 turn of action, and have to wait for their delay counter to fill up before taking their next action.
A creature with 975 speed, meanwhile, (which is functionally where most dwarves fall, thanks to low agility) has to wait 9 turns, then rolls a random chance to see if it goes that turn or not - with a 75% chance of having to wait another turn.
Agility acts as a multiplier on speed - if someone had only 400 speed/delay to begin with, and another creature had 900 speed/delay, then regardless of whether it was low or high agility, as long as they had the same amount of agility (and equipment), the 400 speed/delay creature would act twice as fast. A creature with 0 speed/delay has no benefit or penalty from agility at all.