Mobility item is consumed when sending your troops on marches across the world map.
Each mission requires a specific amount of this item.

How to obtain this item:
1. From the Event Center:

-Go to the "Event Center" building inside your city.
-Tap on the "Mobility " icon.
-Claim to restore Mobility items from "the Mobility Supply crate," which is automatically generated at regular intervals.

2. From the Player Info Screen:
-Tap on the player icon on the upper left side of the game screen.
-Click the "+" sign next to the mobility bar.

From there, you can check your balance and explore other ways to obtain more of this item.