This is the 386th most frequent Finnish word.
sellaista
"Such" or "that kind of."
In this sentence, "sellaista" is used to describe an unspecified type of movie that the speaker finds remarkable or noteworthy in some way. It plays the role of an adjective-like descriptor for the noun "elokuvaa" (movie).
En ole koskaan nähnyt sellaista elokuvaa.
I have never seen such a movie.
Here, "sellaista" refers to a type or kind of information or topic that was previously mentioned. It acts as a pronoun reference connecting to earlier conversation context.
Kerro minulle lisää sellaista, mitä kerroit aiemmin.
Tell me more of that kind that you mentioned earlier.
Here, 'sellaista' is partitive case and refers to something previously mentioned or implied that is unusual or specific.