Piso vs. suelo
The Spanish words piso and suelo both refer to floor, however, they are used in different contexts and cannot always be used interchangeably.
Piso
A1Piso generally refers to the floor as a surface inside a building or a flat, similar to the English concept of floor. It can also refer to the level of a building or an apartment (flat).
Estoy limpiando el piso.
(I am cleaning the floor.)
Vivo en un piso en el centro de la ciudad.
(I live in an apartment in the city center.)
El edificio tiene muchos pisos.
(The building has many floors.)
Suelo
A2Suelo means ground, soil, land or floor typically referring to earth's surface. When used for floor, it often regards the outdoor floors like patio floor, ground under your feet when you're outside or the surface that you walk on outside. However, it can also be used broadly for any type of surface you walk on.
Estoy caminando en el suelo de piedra.
(I'm walking on the stone ground.)
El suelo de la selva es muy fértil.
(The soil of the jungle is very fertile.)
El niño está jugando en el suelo.
(The child is playing on the floor.)
Summary
Piso and suelo both translate to floor but they are utilized differently based on context. Piso usually refers to an indoor floor or a level in a building while suelo often translates to outdoor grounds or earth's soil. However, there are circumstances where they might overlap.