Units are expensive! The more effective the unit, the higher the price.
But to avoid spending credits by building a new unit every time it's hit points are lowered below a number that's still effective in battle, you can heal your units.
To heal a unit, move it on a building that can heal (not all buildings provide this)
Leave the unit on that building for the remainder of the turn.
When a new turn starts, your unit will be healed for a certain number of hit points.
This can be used effectively when defending a position.