The same as in english.
The "S" on an elevator in Spanish stands for "Suelo," which means "ground" or "floor." It indicates the main floor or ground level of a building.
floor or ground
It generally means "plant", but can have different meanings in certain expressions. For example, "planta baja" means "ground floor".
the ground floor is called 'le rez-de-chaussée' in French.
On the floor/In the background
It can be used as a metaphor for a compromise.
"Piso" can have different meanings depending on the context. In Spanish, "piso" can mean floor or apartment. In Filipino, "piso" refers to the currency unit equivalent to 100 centavos.
floor in Spanish is piso
The ground floor is at ground level so the floor immediately above it must be the first floor.
"Primer piso" means "first floor."
Dropping your keys on the floor simply means accidentally letting them fall from your hand to the ground.
If ground is to floor as scare is to fright.