Possible interpretations:
* Piedras platas: This could be a possible correct spelling, meaning "flat stones". "Piedras" means "stones" and "platas" could be a misspelling of "planas," which means "flat."
* Piedras saladas: This would mean "salty stones." "Saladas" means "salty."
Without more context, it's impossible to determine the intended meaning.
If you could provide more information, such as the sentence where you encountered this phrase, I might be able to give you a more accurate translation.