This is the 563rd most frequent Czech word.
dnešní
"Today's" or "current."
Here, 'dnešní' is used as an adjective modifying 'počasí' (weather), indicating that the weather belongs to or refers to today.
Dnešní počasí je opravdu krásné.
Today's weather is really beautiful.
In this sentence, 'dnešní' modifies 'lekce' (lesson), specifying the lesson that occurred or is associated with today.
Dnešní lekce byla velmi zajímavá.
Today's lesson was very interesting.
Here, 'dnešní' describes 'událost' (event), stating that the event took place or is related to today.