Speed is the rate at which units can move across the field.
It is often determined by the size of the units such as small units being fast and larger ones being slow. There are other factors that affect it such as technology and special abilities. Some abilities such as Charge (Zealots) and Afterburners (Medivac dropship) increase speed. While abilities such as Ensnare (Queens) and Concussive shells (Marauders) decrease it.
For more information see Speed on the parent site.