This is the 709th most frequent Croatian word.
programu
"Programu" in Croatian means "to the program" or refers to the dative/locative singular form of "program" (program).
The word 'programu' is used in the locative case in this sentence, indicating participation in something.
Sudjelovali smo u edukativnom programu.
We participated in an educational program.
In this context, 'programu' is also in the locative case, describing the nature of the program.
Radilo se o iznimno korisnom programu.
It was an extremely useful program.
'Programu' here is in the locative case, illustrating that something contained an error.