Autonomy, also known as operational range in Wargame: European Escalation, is an attribute of a unit. Autonomy generally means the distance the unit can go at full fuel capacity until it runs out of fuel.
In-game however, this value translates to how many seconds a unit can move before it runs out of fuel. E.G the both the Strv 103B and the Strv 104 both have 400 km in autonomy, therefore they both can drive for 400 seconds before running out of fuel. Driving across a field or on a road makes no difference.
Infantry units have full autonomy, but move slowly on their own.