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
'Piso' 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
'Suelo' 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.