Movement costs are a terrain stat which effect how easily a unit can cross them. Each unit has a set amount of movement points, which indicates how far they can move. When a unit crosses terrain, the movement cost is subtracted from their movement points. If a unit does not have enough movement points for the equation to be 0 or higher, it cannot cross that square. If a unit reaches a situation where this happens on all nearby squares it hasn't yet moved across or it runs out of movement points, it cannot move any further in that direction and it will have to end its turn or move elsewhere instead.