This is the 25th most frequent Ukrainian word.
це
"Це" means "this" or "it" in English.
Here, 'це' serves as the subject of the sentence, identifying 'my book' as the object being referenced.
Це моя книга.
This is my book.
In this instance, 'це' is used in an object clause to refer to the general idea or statement that is being emphasized as important.
Він сказав, що це важливо.
He said that this is important.
'Це' here is part of the interrogative phrase, asking for identification or explanation of an unspecified object or concept.