This is the 9th most frequent Czech word.
za
"Za" in Czech means "behind," "for," or "after," depending on context.
The word "za" is used here as a preposition meaning "behind" to indicate a location.
Šel jsem za domem a našel krásnou zahradu.
I went behind the house and found a beautiful garden.
Here, "za" is a preposition meaning "for" to express substitution or exchange.
Vyměnil jsem staré knihy za nové.
I exchanged old books for new ones.
In this sentence, "za" functions as a preposition meaning "for" to denote gratitude for something received.