Speed is a unit stat that will determine how many seconds your troops will take to move one tile on the map. To check each unit's speed at any level see the Official Help Database page for that unit or the wiki page for Units for stats at level 1 and level 10.
When you send out a troop, the average speed of the units in that troop is calculated and used to determine how many seconds the troop will take to travel 1 tile. If war machines such as Helepoli or Catapults are included in the troop, they will slow down the rest of the troop to their speed rather than using the group average. The function to determine seconds per tile is:
spt = 3600 / (45 + (speed * 5)) rounded to the nearest whole second