There are different types of map fields in the World.
Some of them apply Effects to units. Like slowing down or damaging them.
Earth[]
Nothing special about it.
Grass[]
Nothing special
Nothing special about it.
Sand[]
Slows units down down a little bit.
Water[]
Slows units down.
Lava[]
Slows units down down like water and adds Damage and a Hurt Effect to them.