This is the 12th most frequent Czech word.
který
"Which" or "that" (used as a relative pronoun).
The word 'který' is used as a relative pronoun introducing a subordinate clause describing the subject 'dům' (house).
To je dům, který postavil můj dědeček.
This is the house that my grandfather built.
The word 'který' functions as an interrogative pronoun asking for a specific selection or choice among options.
Nevím, který film chci vidět.
I don't know which movie I want to see.
In this example, 'který' introduces a descriptive or defining clause associated with 'den' (day), specifying it.