The movement speed of the party on the global map. The base mobility of an army is determined by the speed of its slowest unit. It can be increased by special buildings and by hero skills.
Each point in mobility corresponds to crossing of one province if there are no penalties. The province terrain type affects the mobility cost of passing provinces. Even one point in the specific Terrain Knowledge skill ignore these penalties.
If the province is not yours, it cannot be crossed in more than one turn regardless of how high the mobility is.