Learn Spanish (Latin America): La roca que piso es resbalosa.

In English, we say 'step on', but the Spanish verb pisar already means 'to step on' or 'to tread on' all by itself. So, que piso translates perfectly to 'that I step on' without needing an extra preposition like en or sobre.

I thought piso meant 'floor'. Is it acting as a noun here?

Piso is a very common noun meaning 'floor', but here it is the 'yo' (I) form of the verb pisar, which means 'to step' or 'to step on'. Context tells you it's a verb because it comes right after the relative pronoun que (that) to describe the action happening to the rock.

Why do we use es instead of está for 'is slippery'?

While a rock being temporarily wet from rain would use estar, the physical texture of this specific rock is inherently smooth or slick. When describing the natural, built-in physical characteristics of an object, Spanish uses the verb ser (here, es).

Is resbalosa the standard word for 'slippery'?

In Latin America, resbaloso and its feminine form resbalosa are the most common, everyday words for 'slippery'. You might also encounter resbaladizo, which means the exact same thing but is more typical in Spain or in formal writing.