This is the 686th most frequent Polish word.
cieszyć
"To enjoy" or "to rejoice."
Here, 'cieszyć się' means 'to be happy' or 'to enjoy', expressing joy about an event.
Cieszę się, że dziś świeci słońce.
I'm happy that the sun is shining today.
Here, 'cieszyć się na' conveys anticipation or looking forward to something.
Dzieci cieszą się na wycieczkę do zoo.
The children are looking forward to the trip to the zoo.
In this sentence, 'cieszyć się' is used in a figurative sense meaning 'to enjoy' in terms of popularity.