Units can be assembled into armies and moved from Settlement to Settlement for a variety of purposes, mostly for offensive missions against enemy Settlements or for the purpose of defending their Empire's Settlements. Unit movement costs Food and takes time. Both of these factors are affected by a Unit's Movement Cost and Speed, which can be viewed in Units.
Any and all Unit movements can be viewed at the Barracks of the Settlement where they were dispatched from. There, Leaders can track how much longer an army has before it reaches its destination.