This is the 9453rd most frequent Polish word.
warta
"Warta" means "guard" or "watch" in English, often referring to a duty or shift of guarding something.
Here, 'Warta' is used as a proper noun, specifically the name of a river in Poland.
Warta to nazwa polskiej rzeki.
Warta is the name of a Polish river.
In this sentence, 'wartę' (a form of 'warta') refers to a shift or duty period of a guard or sentinel.
Strażnik pełnił wartę przez całą noc.
The guard was on duty throughout the night.
Here, 'warta' is part of the phrase 'warta zachodu', meaning 'worth the effort', showing its idiomatic usage.