This is the 57th most frequent Romanian word.
ceea
The Romanian word "ceea" means "that" (used as a demonstrative pronoun).
In this sentence, "ceea" is used as a pronoun, specifically to introduce a relative clause.
Ceea ce facem acum va fi important mai târziu.
What we are doing now will matter later.
Here, "ceea" is part of a phrase that functions as a direct object to the verb "a văzut".
Am văzut ceea ce a spus profesorul astăzi.
I saw what the teacher said today.
In this instance, "ceea" is used to point to a concept or idea mentioned in the clause that follows.