This is the 507th most frequent Czech word.
seriálu
The Czech word "seriálu" means "of the series" or "of a TV show/serial" (genitive singular).
Here, 'seriálu' is in the singular genitive case, meaning 'of the series', used to specify the show that was watched.
Viděl jsem včera nový díl populárního seriálu.
I watched yesterday a new episode of the popular TV series.
Again, 'seriálu' is in the genitive case, indicating 'of the series', in the context of discussing its development.
Diskutovali jsme o průběhu seriálu.
We discussed the progress of the TV series.
In this sentence, 'seriálu' is also in the genitive case, referring to 'of the series', specifying what the theme belongs to.